diff --git a/src/AbstractCommand.cc b/src/AbstractCommand.cc
index 884fa037..687e7881 100644
--- a/src/AbstractCommand.cc
+++ b/src/AbstractCommand.cc
@@ -237,8 +237,7 @@ bool AbstractCommand::execute()
       if (req_ && fileEntry_->getLength() > 0 &&
           e_->getRequestGroupMan()->getMaxOverallDownloadSpeedLimit() == 0 &&
           requestGroup_->getMaxDownloadSpeedLimit() == 0 &&
-          serverStatTimer_.difference(global::wallclock()) >=
-              std::chrono::seconds(10)) {
+          serverStatTimer_.difference(global::wallclock()) >= 10_s) {
         serverStatTimer_ = global::wallclock();
         std::vector<std::pair<size_t, std::string>> usedHosts;
         if (getOption()->getAsBool(PREF_SELECT_LEAST_USED_HOST)) {
diff --git a/src/AbstractHttpServerResponseCommand.cc b/src/AbstractHttpServerResponseCommand.cc
index 07ae91be..057e74fa 100644
--- a/src/AbstractHttpServerResponseCommand.cc
+++ b/src/AbstractHttpServerResponseCommand.cc
@@ -119,8 +119,7 @@ bool AbstractHttpServerResponseCommand::execute()
     afterSend(httpServer_, e_);
     return true;
   } else {
-    if (timeoutTimer_.difference(global::wallclock()) >=
-        std::chrono::seconds(30)) {
+    if (timeoutTimer_.difference(global::wallclock()) >= 30_s) {
       A2_LOG_INFO(fmt("CUID#%" PRId64
                       " - HttpServer: Timeout while trasmitting response.",
                       getCuid()));
diff --git a/src/AdaptiveURISelector.cc b/src/AdaptiveURISelector.cc
index 3c9c8b3a..ebe7bd0b 100644
--- a/src/AdaptiveURISelector.cc
+++ b/src/AdaptiveURISelector.cc
@@ -97,7 +97,7 @@ std::string AdaptiveURISelector::select
 }
 
 namespace {
-constexpr auto MAX_TIMEOUT = std::chrono::seconds(60);
+constexpr auto MAX_TIMEOUT = 60_s;
 } // namespace
 
 void AdaptiveURISelector::mayRetryWithIncreasedTimeout(FileEntry* fileEntry)
diff --git a/src/BtAnnounce.cc b/src/BtAnnounce.cc
index 1d77e4d6..7f1cce2b 100644
--- a/src/BtAnnounce.cc
+++ b/src/BtAnnounce.cc
@@ -54,6 +54,6 @@ const std::string BtAnnounce::PEERS("peers");
 
 const std::string BtAnnounce::PEERS6("peers6");
 
-constexpr std::chrono::seconds BtAnnounce::DEFAULT_ANNOUNCE_INTERVAL;
+constexpr std::chrono::minutes BtAnnounce::DEFAULT_ANNOUNCE_INTERVAL;
 
 } // namespace aria2
diff --git a/src/BtAnnounce.h b/src/BtAnnounce.h
index 2ee2131f..21688bb7 100644
--- a/src/BtAnnounce.h
+++ b/src/BtAnnounce.h
@@ -41,6 +41,7 @@
 #include <memory>
 
 #include "a2time.h"
+#include "a2functional.h"
 
 namespace aria2 {
 
@@ -138,7 +139,7 @@ public:
 
   static const std::string PEERS6;
 
-  constexpr static auto DEFAULT_ANNOUNCE_INTERVAL = std::chrono::seconds(120);
+  constexpr static auto DEFAULT_ANNOUNCE_INTERVAL = 2_min;
 };
 
 } // namespace aria2
diff --git a/src/BtLeecherStateChoke.cc b/src/BtLeecherStateChoke.cc
index aab46561..a7ddd58f 100644
--- a/src/BtLeecherStateChoke.cc
+++ b/src/BtLeecherStateChoke.cc
@@ -56,9 +56,9 @@ BtLeecherStateChoke::PeerEntry::PeerEntry(const std::shared_ptr<Peer>& peer)
   : peer_(peer),
     downloadSpeed_(peer->calculateDownloadSpeed()),
     // peer must be interested to us and sent block in the last 30 seconds
-    regularUnchoker_(peer->peerInterested() &&
-                     peer->getLastDownloadUpdate().difference(
-                         global::wallclock()) < std::chrono::seconds(30))
+    regularUnchoker_(
+        peer->peerInterested() &&
+        peer->getLastDownloadUpdate().difference(global::wallclock()) < 30_s)
 {
 }
 
diff --git a/src/BtSeederStateChoke.cc b/src/BtSeederStateChoke.cc
index 4e7c610f..c192f896 100644
--- a/src/BtSeederStateChoke.cc
+++ b/src/BtSeederStateChoke.cc
@@ -53,7 +53,7 @@ BtSeederStateChoke::BtSeederStateChoke()
 BtSeederStateChoke::~BtSeederStateChoke() {}
 
 namespace {
-constexpr auto TIME_FRAME = std::chrono::seconds(20);
+constexpr auto TIME_FRAME = 20_s;
 } // namespace
 
 BtSeederStateChoke::PeerEntry::PeerEntry
diff --git a/src/BtSetup.cc b/src/BtSetup.cc
index 7e9ec591..4cf4a6b5 100644
--- a/src/BtSetup.cc
+++ b/src/BtSetup.cc
@@ -126,8 +126,7 @@ void BtSetup::setup(std::vector<std::unique_ptr<Command>>& commands,
   }
   {
     auto c = make_unique<ActivePeerConnectionCommand>(
-        e->newCUID(), requestGroup, e,
-        std::chrono::seconds(metadataGetMode ? 2 : 10));
+        e->newCUID(), requestGroup, e, metadataGetMode ? 2_s : 10_s);
     c->setBtRuntime(btRuntime);
     c->setPieceStorage(pieceStorage);
     c->setPeerStorage(peerStorage);
diff --git a/src/BtStopDownloadCommand.cc b/src/BtStopDownloadCommand.cc
index ba3ebcaf..74b3b89d 100644
--- a/src/BtStopDownloadCommand.cc
+++ b/src/BtStopDownloadCommand.cc
@@ -51,7 +51,7 @@ BtStopDownloadCommand::BtStopDownloadCommand
  RequestGroup* requestGroup,
  DownloadEngine* e,
  std::chrono::seconds timeout)
-  : TimeBasedCommand(cuid, e, std::chrono::seconds(1)),
+  : TimeBasedCommand(cuid, e, 1_s),
     requestGroup_(requestGroup),
     timeout_(std::move(timeout))
 {}
diff --git a/src/ConsoleStatCalc.cc b/src/ConsoleStatCalc.cc
index d90e6284..a633469e 100644
--- a/src/ConsoleStatCalc.cc
+++ b/src/ConsoleStatCalc.cc
@@ -318,7 +318,7 @@ ConsoleStatCalc::calculateStat(const DownloadEngine* e)
   }
   ColorizedStream o;
   if(e->getRequestGroupMan()->countRequestGroup() > 0) {
-    if((summaryInterval_ > std::chrono::seconds(0)) &&
+    if((summaryInterval_ > 0_s) &&
        lastSummaryNotified_.difference(global::wallclock())+
        A2_DELTA_MILLIS >= summaryInterval_*1000) {
       lastSummaryNotified_ = global::wallclock();
diff --git a/src/DHTConstants.h b/src/DHTConstants.h
index a59b1db7..8d84046c 100644
--- a/src/DHTConstants.h
+++ b/src/DHTConstants.h
@@ -37,6 +37,7 @@
 
 #include "common.h"
 #include "TimerA2.h"
+#include "a2functional.h"
 
 // Increment this if major improvements or bug fixes are made in DHT
 // code. This is 2 bytes unsigned integer.
@@ -48,21 +49,25 @@
 
 #define DHT_TOKEN_LENGTH 4
 
+namespace aria2 {
+
 // See --dht-message-timeout option.
-constexpr auto DHT_MESSAGE_TIMEOUT = std::chrono::seconds(10);
+constexpr auto DHT_MESSAGE_TIMEOUT = 10_s;
 
-constexpr auto DHT_NODE_CONTACT_INTERVAL = std::chrono::minutes(15);
+constexpr auto DHT_NODE_CONTACT_INTERVAL = 15_min;
 
-constexpr auto DHT_BUCKET_REFRESH_INTERVAL = std::chrono::minutes(15);
+constexpr auto DHT_BUCKET_REFRESH_INTERVAL = 15_min;
 
-constexpr auto DHT_BUCKET_REFRESH_CHECK_INTERVAL = std::chrono::minutes(5);
+constexpr auto DHT_BUCKET_REFRESH_CHECK_INTERVAL = 5_min;
 
-constexpr auto DHT_PEER_ANNOUNCE_PURGE_INTERVAL = std::chrono::minutes(30);
+constexpr auto DHT_PEER_ANNOUNCE_PURGE_INTERVAL = 30_min;
 
-constexpr auto DHT_PEER_ANNOUNCE_INTERVAL = std::chrono::minutes(15);
+constexpr auto DHT_PEER_ANNOUNCE_INTERVAL = 15_min;
 
-constexpr auto DHT_PEER_ANNOUNCE_CHECK_INTERVAL = std::chrono::minutes(5);
+constexpr auto DHT_PEER_ANNOUNCE_CHECK_INTERVAL = 5_min;
 
-constexpr auto DHT_TOKEN_UPDATE_INTERVAL = std::chrono::minutes(10);
+constexpr auto DHT_TOKEN_UPDATE_INTERVAL = 10_min;
+
+} // namespace aria2
 
 #endif // D_DHT_CONSTANTS_H
diff --git a/src/DHTGetPeersCommand.cc b/src/DHTGetPeersCommand.cc
index c992ae1b..da52ce8e 100644
--- a/src/DHTGetPeersCommand.cc
+++ b/src/DHTGetPeersCommand.cc
@@ -55,13 +55,13 @@ namespace aria2 {
 
 namespace {
 
-constexpr auto GET_PEER_INTERVAL = std::chrono::minutes(15);
+constexpr auto GET_PEER_INTERVAL = 15_min;
 // Interval when the size of peer list is low.
-constexpr auto GET_PEER_INTERVAL_LOW = std::chrono::minutes(5);
+constexpr auto GET_PEER_INTERVAL_LOW = 5_min;
 // Interval when the peer list is empty.
-constexpr auto GET_PEER_INTERVAL_ZERO = std::chrono::minutes(1);
+constexpr auto GET_PEER_INTERVAL_ZERO = 1_min;
 // Interval for retry.
-constexpr auto GET_PEER_INTERVAL_RETRY = std::chrono::seconds(5);
+constexpr auto GET_PEER_INTERVAL_RETRY = 5_s;
 // Maximum retries. Try more than 5 to drop bad node.
 const int MAX_RETRIES = 10;
 
diff --git a/src/DHTMessageDispatcherImpl.cc b/src/DHTMessageDispatcherImpl.cc
index 027fb2ae..9f4a92c5 100644
--- a/src/DHTMessageDispatcherImpl.cc
+++ b/src/DHTMessageDispatcherImpl.cc
@@ -92,7 +92,7 @@ bool DHTMessageDispatcherImpl::sendMessage(DHTMessageEntry* entry)
     // DHTTask(such as DHTAbstractNodeLookupTask) don't finish
     // forever.
     if(!entry->message->isReply()) {
-      tracker_->addMessage(entry->message.get(), std::chrono::seconds(0),
+      tracker_->addMessage(entry->message.get(), 0_s,
                            std::move(entry->callback));
     }
   }
diff --git a/src/DHTSetup.cc b/src/DHTSetup.cc
index d369d935..64dbd596 100644
--- a/src/DHTSetup.cc
+++ b/src/DHTSetup.cc
@@ -235,7 +235,7 @@ std::vector<std::unique_ptr<Command>> DHTSetup::setup
     }
     {
       auto command = make_unique<DHTAutoSaveCommand>(e->newCUID(), e, family,
-                                                     std::chrono::minutes(30));
+                                                     30_min);
       command->setLocalNode(localNode);
       command->setRoutingTable(routingTable.get());
       tempCommands.push_back(std::move(command));
diff --git a/src/DefaultBtAnnounce.cc b/src/DefaultBtAnnounce.cc
index 34042525..a32b1905 100644
--- a/src/DefaultBtAnnounce.cc
+++ b/src/DefaultBtAnnounce.cc
@@ -64,7 +64,7 @@ DefaultBtAnnounce::DefaultBtAnnounce
     prevAnnounceTimer_(Timer::zero()),
     interval_(DEFAULT_ANNOUNCE_INTERVAL),
     minInterval_(DEFAULT_ANNOUNCE_INTERVAL),
-    userDefinedInterval_(std::chrono::seconds(0)),
+    userDefinedInterval_(0_s),
     complete_(0),
     incomplete_(0),
     announceList_(bittorrent::getTorrentAttrs(downloadContext)->announceList),
diff --git a/src/DefaultBtInteractive.cc b/src/DefaultBtInteractive.cc
index 24dbbd78..fc640e91 100644
--- a/src/DefaultBtInteractive.cc
+++ b/src/DefaultBtInteractive.cc
@@ -255,7 +255,7 @@ void DefaultBtInteractive::decideChoking() {
 }
 
 namespace {
-constexpr auto MAX_HAVE_DELAY_SEC = std::chrono::seconds(10);
+constexpr auto MAX_HAVE_DELAY_SEC = 10_s;
 } // namespace
 
 void DefaultBtInteractive::checkHave() {
@@ -434,7 +434,7 @@ void DefaultBtInteractive::sendPendingMessage() {
 }
 
 namespace {
-constexpr auto FLOODING_CHECK_INTERVAL = std::chrono::seconds(5);
+constexpr auto FLOODING_CHECK_INTERVAL = 5_s;
 } // namespace
 
 void DefaultBtInteractive::detectMessageFlooding() {
@@ -529,8 +529,7 @@ void DefaultBtInteractive::doInteractionProcessing() {
           dispatcher_->addMessageToQueue(std::move(i));
         }
       }
-      if(perSecTimer_.difference(global::wallclock()) >=
-         std::chrono::seconds(1)) {
+      if(perSecTimer_.difference(global::wallclock()) >= 1_s) {
         perSecTimer_ = global::wallclock();
         // Drop timeout request after queuing message to give a chance
         // to other connection to request piece.
@@ -548,8 +547,7 @@ void DefaultBtInteractive::doInteractionProcessing() {
     checkActiveInteraction();
     decideChoking();
     detectMessageFlooding();
-    if(perSecTimer_.difference(global::wallclock()) >=
-       std::chrono::seconds(1)) {
+    if(perSecTimer_.difference(global::wallclock()) >= 1_s) {
       perSecTimer_ = global::wallclock();
       dispatcher_->checkRequestSlotAndDoNecessaryThing();
     }
diff --git a/src/DefaultPeerStorage.cc b/src/DefaultPeerStorage.cc
index f320994b..b3cade14 100644
--- a/src/DefaultPeerStorage.cc
+++ b/src/DefaultPeerStorage.cc
@@ -197,8 +197,7 @@ bool DefaultPeerStorage::isBadPeer(const std::string& ipaddr)
 
 void DefaultPeerStorage::addBadPeer(const std::string& ipaddr)
 {
-  if(lastBadPeerCleaned_.difference(global::wallclock()) >=
-     std::chrono::hours(1)) {
+  if(lastBadPeerCleaned_.difference(global::wallclock()) >= 1_h) {
     for(auto i = std::begin(badPeers_); i != std::end(badPeers_);) {
       if((*i).second <= global::wallclock()) {
         A2_LOG_DEBUG(fmt("Purge %s from bad peer", (*i).first.c_str()));
@@ -284,7 +283,7 @@ void DefaultPeerStorage::returnPeer(const std::shared_ptr<Peer>& peer)
 
 bool DefaultPeerStorage::chokeRoundIntervalElapsed()
 {
-  constexpr auto CHOKE_ROUND_INTERVAL = std::chrono::seconds(10);
+  constexpr auto CHOKE_ROUND_INTERVAL = 10_s;
   if(pieceStorage_->downloadFinished()) {
     return seederStateChoke_->getLastRound().
       difference(global::wallclock()) >= CHOKE_ROUND_INTERVAL;
diff --git a/src/DownloadEngine.cc b/src/DownloadEngine.cc
index 377befe2..522c1b2e 100644
--- a/src/DownloadEngine.cc
+++ b/src/DownloadEngine.cc
@@ -91,7 +91,7 @@ volatile sig_atomic_t globalHaltRequested = 0;
 } // namespace global
 
 namespace {
-constexpr auto DEFAULT_REFRESH_INTERVAL = std::chrono::seconds(1);
+constexpr auto DEFAULT_REFRESH_INTERVAL = 1_s;
 } // namespace
 
 DownloadEngine::DownloadEngine(std::unique_ptr<EventPoll> eventPoll)
@@ -312,8 +312,7 @@ void DownloadEngine::poolSocket(const std::string& key,
   std::multimap<std::string, SocketPoolEntry>::value_type p(key, entry);
   socketPool_.insert(p);
 
-  if(lastSocketPoolScan_.difference(global::wallclock()) <
-     std::chrono::minutes(1)) {
+  if(lastSocketPoolScan_.difference(global::wallclock()) < 1_min) {
     return;
   }
   std::multimap<std::string, SocketPoolEntry> newPool;
diff --git a/src/DownloadEngine.h b/src/DownloadEngine.h
index f6febb29..749092a2 100644
--- a/src/DownloadEngine.h
+++ b/src/DownloadEngine.h
@@ -269,24 +269,24 @@ public:
                   const std::string& proxyhost, uint16_t proxyport,
                   const std::shared_ptr<SocketCore>& sock,
                   const std::string& options,
-                  std::chrono::seconds timeout = std::chrono::seconds(15));
+                  std::chrono::seconds timeout = 15_s);
 
   void poolSocket(const std::shared_ptr<Request>& request,
                   const std::string& username,
                   const std::shared_ptr<Request>& proxyRequest,
                   const std::shared_ptr<SocketCore>& socket,
                   const std::string& options,
-                  std::chrono::seconds timeout = std::chrono::seconds(15));
+                  std::chrono::seconds timeout = 15_s);
 
   void poolSocket(const std::string& ipaddr, uint16_t port,
                   const std::string& proxyhost, uint16_t proxyport,
                   const std::shared_ptr<SocketCore>& sock,
-                  std::chrono::seconds timeout = std::chrono::seconds(15));
+                  std::chrono::seconds timeout = 15_s);
 
   void poolSocket(const std::shared_ptr<Request>& request,
                   const std::shared_ptr<Request>& proxyRequest,
                   const std::shared_ptr<SocketCore>& socket,
-                  std::chrono::seconds timeout = std::chrono::seconds(15));
+                  std::chrono::seconds timeout = 15_s);
 
   std::shared_ptr<SocketCore> popPooledSocket
   (const std::string& ipaddr,
diff --git a/src/DownloadEngineFactory.cc b/src/DownloadEngineFactory.cc
index fe87507c..6f07c2f6 100644
--- a/src/DownloadEngineFactory.cc
+++ b/src/DownloadEngineFactory.cc
@@ -175,8 +175,8 @@ DownloadEngineFactory::newDownloadEngine
         e->newCUID(), e.get(),
         std::chrono::seconds(op->getAsInt(PREF_SAVE_SESSION_INTERVAL))));
   }
-  e->addRoutineCommand(make_unique<HaveEraseCommand>
-                       (e->newCUID(), e.get(), std::chrono::seconds(10)));
+  e->addRoutineCommand(
+      make_unique<HaveEraseCommand>(e->newCUID(), e.get(), 10_s));
   {
     auto stopSec = op->getAsInt(PREF_STOP);
     if(stopSec > 0) {
diff --git a/src/FileEntry.cc b/src/FileEntry.cc
index 2ab1291c..35b660e8 100644
--- a/src/FileEntry.cc
+++ b/src/FileEntry.cc
@@ -214,7 +214,7 @@ FileEntry::getRequest
 }
 
 namespace {
-constexpr auto startupIdleTime = std::chrono::seconds(10);
+constexpr auto startupIdleTime = 10_s;
 } // namespace
 
 std::shared_ptr<Request>
diff --git a/src/HaveEraseCommand.cc b/src/HaveEraseCommand.cc
index ceca170e..b158c7eb 100644
--- a/src/HaveEraseCommand.cc
+++ b/src/HaveEraseCommand.cc
@@ -63,7 +63,7 @@ void HaveEraseCommand::process()
   for(auto & group : groups) {
     const auto& ps = group->getPieceStorage();
     if(ps) {
-      ps->removeAdvertisedPiece(std::chrono::seconds(5));
+      ps->removeAdvertisedPiece(5_s);
     }
   }
 }
diff --git a/src/HttpServerBodyCommand.cc b/src/HttpServerBodyCommand.cc
index 7fb715c2..dea078a5 100644
--- a/src/HttpServerBodyCommand.cc
+++ b/src/HttpServerBodyCommand.cc
@@ -149,7 +149,7 @@ void HttpServerBodyCommand::addHttpServerResponseCommand(bool delayed)
     make_unique<HttpServerResponseCommand>(getCuid(), httpServer_, e_, socket_);
   if (delayed) {
     e_->addCommand(make_unique<DelayedCommand>(
-        getCuid(), e_, std::chrono::seconds(1), std::move(resp), true));
+        getCuid(), e_, 1_s, std::move(resp), true));
     return;
   }
 
@@ -320,8 +320,7 @@ bool HttpServerBodyCommand::execute()
         return false;
       }
     } else {
-      if(timeoutTimer_.difference(global::wallclock()) >=
-         std::chrono::seconds(30)) {
+      if(timeoutTimer_.difference(global::wallclock()) >= 30_s) {
         A2_LOG_INFO("HTTP request body timeout.");
         return true;
       } else {
diff --git a/src/HttpServerCommand.cc b/src/HttpServerCommand.cc
index f2fdb137..bab3ab29 100644
--- a/src/HttpServerCommand.cc
+++ b/src/HttpServerCommand.cc
@@ -253,8 +253,7 @@ bool HttpServerCommand::execute()
         return true;
       }
     } else {
-      if(timeoutTimer_.difference(global::wallclock()) >=
-         std::chrono::seconds(30)) {
+      if(timeoutTimer_.difference(global::wallclock()) >= 30_s) {
         A2_LOG_INFO("HTTP request timeout.");
         return true;
       } else {
diff --git a/src/LpdMessageDispatcher.h b/src/LpdMessageDispatcher.h
index 42cac417..868f3671 100644
--- a/src/LpdMessageDispatcher.h
+++ b/src/LpdMessageDispatcher.h
@@ -58,7 +58,7 @@ public:
   LpdMessageDispatcher(
       const std::string& infoHash, uint16_t port,
       const std::string& multicastAddr, uint16_t multicastPort,
-      std::chrono::seconds interval = std::chrono::minutes(5));
+      std::chrono::seconds interval = 5_min);
 
   ~LpdMessageDispatcher();
 
diff --git a/src/RequestGroup.cc b/src/RequestGroup.cc
index d43f4af3..cb23c9d7 100644
--- a/src/RequestGroup.cc
+++ b/src/RequestGroup.cc
@@ -978,7 +978,7 @@ void RequestGroup::releaseRuntimeResource(DownloadEngine* e)
   peerStorage_ = nullptr;
 #endif // ENABLE_BITTORRENT
   if(pieceStorage_) {
-    pieceStorage_->removeAdvertisedPiece(std::chrono::seconds(0));
+    pieceStorage_->removeAdvertisedPiece(0_s);
   }
   // Don't reset segmentMan_ and pieceStorage_ here to provide
   // progress information via RPC
diff --git a/src/RpcMethodImpl.cc b/src/RpcMethodImpl.cc
index cdd51375..f8a043b1 100644
--- a/src/RpcMethodImpl.cc
+++ b/src/RpcMethodImpl.cc
@@ -1306,8 +1306,8 @@ std::unique_ptr<ValueBase> goingShutdown
 {
   // Schedule shutdown after 3seconds to give time to client to
   // receive RPC response.
-  e->addRoutineCommand(make_unique<TimedHaltCommand>(
-      e->newCUID(), e, std::chrono::seconds(3), forceHalt));
+  e->addRoutineCommand(
+      make_unique<TimedHaltCommand>(e->newCUID(), e, 3_s, forceHalt));
   A2_LOG_INFO("Scheduled shutdown in 3 seconds.");
   return createOKResponse();
 }
diff --git a/src/SpeedCalc.cc b/src/SpeedCalc.cc
index 4416c2af..c1ce3492 100644
--- a/src/SpeedCalc.cc
+++ b/src/SpeedCalc.cc
@@ -42,7 +42,7 @@
 namespace aria2 {
 
 namespace {
-constexpr auto WINDOW_TIME = std::chrono::seconds(15);
+constexpr auto WINDOW_TIME = 15_s;
 } // namespace
 
 SpeedCalc::SpeedCalc()
@@ -94,7 +94,7 @@ void SpeedCalc::update(size_t bytes)
   removeStaleTimeSlot(now);
   if(timeSlots_.empty() ||
      std::chrono::duration_cast<std::chrono::seconds>(
-         timeSlots_.back().first.difference(now)) >= std::chrono::seconds(1)) {
+         timeSlots_.back().first.difference(now)) >= 1_s) {
     timeSlots_.push_back(std::make_pair(now, bytes));
   } else {
     timeSlots_.back().second += bytes;
diff --git a/src/TimerA2.cc b/src/TimerA2.cc
index 7c16ba9d..a07a16be 100644
--- a/src/TimerA2.cc
+++ b/src/TimerA2.cc
@@ -55,7 +55,7 @@ Timer::Clock::duration Timer::difference() const
 {
   auto now = Clock::now();
   if (now < tp_) {
-    return Timer::Clock::duration(std::chrono::seconds(0));
+    return Timer::Clock::duration(0_s);
   }
 
   return now - tp_;
@@ -64,7 +64,7 @@ Timer::Clock::duration Timer::difference() const
 Timer::Clock::duration Timer::difference(const Timer& timer) const
 {
   if (timer.tp_ < tp_) {
-    return Timer::Clock::duration(std::chrono::seconds(0));
+    return Timer::Clock::duration(0_s);
   }
 
   return timer.tp_ - tp_;
diff --git a/src/TimerA2.h b/src/TimerA2.h
index 3ca4722c..e229dcd7 100644
--- a/src/TimerA2.h
+++ b/src/TimerA2.h
@@ -40,6 +40,7 @@
 #include <chrono>
 
 #include "a2time.h"
+#include "a2functional.h"
 
 namespace aria2 {
 
@@ -92,7 +93,7 @@ public:
 
   const Clock::time_point& getTime() const { return tp_; }
 
-  constexpr static Timer zero() { return Timer(std::chrono::seconds(0)); }
+  constexpr static Timer zero() { return Timer(0_s); }
 
 private:
   Clock::time_point tp_;
diff --git a/src/UDPTrackerClient.cc b/src/UDPTrackerClient.cc
index 41e52f37..11f39add 100644
--- a/src/UDPTrackerClient.cc
+++ b/src/UDPTrackerClient.cc
@@ -370,7 +370,7 @@ struct TimeoutCheck {
   {
     auto t = req->dispatched.difference(now);
     if(req->failCount == 0) {
-      if(t >= std::chrono::seconds(15)) {
+      if(t >= 15_s) {
         switch(req->action) {
         case UDPT_ACT_CONNECT:
           A2_LOG_INFO(fmt("UDPT resend CONNECT to %s:%u transaction_id=%u",
@@ -396,7 +396,7 @@ struct TimeoutCheck {
         return false;
       }
     } else {
-      if(t >= std::chrono::minutes(1)) {
+      if(t >= 1_min) {
         switch(req->action) {
         case UDPT_ACT_CONNECT:
           A2_LOG_INFO(fmt("UDPT timeout CONNECT to %s:%u transaction_id=%u",
@@ -475,7 +475,7 @@ UDPTrackerConnection* UDPTrackerClient::getConnectionId
     return nullptr;
   }
   if((*i).second.state == UDPT_CST_CONNECTED &&
-     (*i).second.lastUpdated.difference(now) > std::chrono::minutes(1)) {
+     (*i).second.lastUpdated.difference(now) > 1_min) {
     connectionIdCache_.erase(i);
     return nullptr;
   } else {
diff --git a/src/UTMetadataRequestTracker.cc b/src/UTMetadataRequestTracker.cc
index 0b517568..cbb20806 100644
--- a/src/UTMetadataRequestTracker.cc
+++ b/src/UTMetadataRequestTracker.cc
@@ -65,7 +65,7 @@ void UTMetadataRequestTracker::remove(size_t index)
 }
 
 namespace {
-constexpr auto TIMEOUT = std::chrono::seconds(20);
+constexpr auto TIMEOUT = 20_s;
 } // namespace
 
 std::vector<size_t> UTMetadataRequestTracker::removeTimeoutEntry()
diff --git a/src/UTPexExtensionMessage.h b/src/UTPexExtensionMessage.h
index 90eff9be..2bdae529 100644
--- a/src/UTPexExtensionMessage.h
+++ b/src/UTPexExtensionMessage.h
@@ -42,6 +42,7 @@
 #include <memory>
 
 #include "a2time.h"
+#include "a2functional.h"
 
 namespace aria2 {
 
@@ -120,7 +121,7 @@ public:
     return maxDroppedPeer_;
   }
 
-  constexpr static auto DEFAULT_INTERVAL = std::chrono::minutes(1);
+  constexpr static auto DEFAULT_INTERVAL = 1_min;
 };
 
 } // namespace aria2
diff --git a/src/WatchProcessCommand.cc b/src/WatchProcessCommand.cc
index f3fa4923..6c58d59d 100644
--- a/src/WatchProcessCommand.cc
+++ b/src/WatchProcessCommand.cc
@@ -70,7 +70,7 @@ WatchProcessCommand::WatchProcessCommand
  DownloadEngine* e,
  unsigned int pid,
  bool forceHalt)
-  : TimeBasedCommand(cuid, e, std::chrono::seconds(1), true),
+  : TimeBasedCommand(cuid, e, 1_s, true),
     pid_(pid),
     forceHalt_(forceHalt)
 {}
diff --git a/src/WebSocketSession.cc b/src/WebSocketSession.cc
index 83a3f7f9..d51e789c 100644
--- a/src/WebSocketSession.cc
+++ b/src/WebSocketSession.cc
@@ -290,8 +290,8 @@ void WebSocketSession::addTextMessage(const std::string& msg, bool delayed)
     auto e = getDownloadEngine();
     auto cuid = command_->getCuid();
     auto c = make_unique<TextMessageCommand>(cuid, command_->getSession(), msg);
-    e->addCommand(make_unique<DelayedCommand>(cuid, e, std::chrono::seconds(1),
-                                              std::move(c), false));
+    e->addCommand(
+        make_unique<DelayedCommand>(cuid, e, 1_s, std::move(c), false));
     return;
   }
 
diff --git a/src/a2functional.h b/src/a2functional.h
index ce419f41..ff0bcb95 100644
--- a/src/a2functional.h
+++ b/src/a2functional.h
@@ -41,6 +41,7 @@
 #include <string>
 #include <algorithm>
 #include <memory>
+#include <chrono>
 
 #include "A2STR.h"
 
@@ -187,6 +188,26 @@ make_unique(size_t size)
   return std::unique_ptr<T>(new typename std::remove_extent<T>::type[size]());
 }
 
+constexpr std::chrono::hours operator"" _h(unsigned long long h)
+{
+  return std::chrono::hours(h);
+}
+
+constexpr std::chrono::minutes operator"" _min(unsigned long long min)
+{
+  return std::chrono::minutes(min);
+}
+
+constexpr std::chrono::seconds operator"" _s(unsigned long long s)
+{
+  return std::chrono::seconds(s);
+}
+
+constexpr std::chrono::milliseconds operator"" _ms(unsigned long long ms)
+{
+  return std::chrono::milliseconds(ms);
+}
+
 } // namespace aria2
 
 #endif // D_A2_FUNCTIONAL_H
diff --git a/test/DHTMessageTrackerEntryTest.cc b/test/DHTMessageTrackerEntryTest.cc
index 316e1e25..67147e66 100644
--- a/test/DHTMessageTrackerEntryTest.cc
+++ b/test/DHTMessageTrackerEntryTest.cc
@@ -43,7 +43,7 @@ void DHTMessageTrackerEntryTest::testMatch()
     DHTMessageTrackerEntry entry(msg1->getRemoteNode(),
                                  msg1->getTransactionID(),
                                  msg1->getMessageType(),
-                                 std::chrono::seconds(30));
+                                 30_s);
 
     CPPUNIT_ASSERT(entry.match(msg1->getTransactionID(),
                                msg1->getRemoteNode()->getIPAddress(),
diff --git a/test/DHTPeerAnnounceEntryTest.cc b/test/DHTPeerAnnounceEntryTest.cc
index 2d518bce..c2531d52 100644
--- a/test/DHTPeerAnnounceEntryTest.cc
+++ b/test/DHTPeerAnnounceEntryTest.cc
@@ -40,7 +40,7 @@ void DHTPeerAnnounceEntryTest::testRemoveStalePeerAddrEntry()
   entry.addPeerAddrEntry(PeerAddrEntry("192.168.0.3", 6883));
   entry.addPeerAddrEntry(PeerAddrEntry("192.168.0.4", 6884, Timer::zero()));
 
-  entry.removeStalePeerAddrEntry(std::chrono::seconds(10));
+  entry.removeStalePeerAddrEntry(10_s);
 
   CPPUNIT_ASSERT_EQUAL((size_t)2, entry.countPeerAddrEntry());
 
diff --git a/test/DefaultBtMessageDispatcherTest.cc b/test/DefaultBtMessageDispatcherTest.cc
index 5d49756a..ce792af7 100644
--- a/test/DefaultBtMessageDispatcherTest.cc
+++ b/test/DefaultBtMessageDispatcherTest.cc
@@ -209,7 +209,7 @@ void DefaultBtMessageDispatcherTest::testCheckRequestSlotAndDoNecessaryThing()
   CPPUNIT_ASSERT(piece->getMissingUnusedBlockIndex(index));
   CPPUNIT_ASSERT_EQUAL((size_t)0, index);
 
-  btMessageDispatcher->setRequestTimeout(std::chrono::minutes(1));
+  btMessageDispatcher->setRequestTimeout(1_min);
   btMessageDispatcher->addOutstandingRequest
     (make_unique<RequestSlot>(0, 0, MY_PIECE_LENGTH, 0, piece));
 
@@ -228,7 +228,7 @@ testCheckRequestSlotAndDoNecessaryThing_timeout() {
   CPPUNIT_ASSERT(piece->getMissingUnusedBlockIndex(index));
   CPPUNIT_ASSERT_EQUAL((size_t)0, index);
 
-  btMessageDispatcher->setRequestTimeout(std::chrono::minutes(1));
+  btMessageDispatcher->setRequestTimeout(1_min);
   auto slot = make_unique<RequestSlot>(0, 0, MY_PIECE_LENGTH, 0, piece);
   // make this slot timeout
   slot->setDispatchedTime(Timer::zero());
@@ -247,7 +247,7 @@ void DefaultBtMessageDispatcherTest::
 testCheckRequestSlotAndDoNecessaryThing_completeBlock() {
   auto piece = std::make_shared<Piece>(0, MY_PIECE_LENGTH);
   piece->completeBlock(0);
-  btMessageDispatcher->setRequestTimeout(std::chrono::minutes(1));
+  btMessageDispatcher->setRequestTimeout(1_min);
   btMessageDispatcher->addOutstandingRequest
     (make_unique<RequestSlot>(0, 0, MY_PIECE_LENGTH, 0, piece));
 
diff --git a/test/RpcMethodTest.cc b/test/RpcMethodTest.cc
index 96127ee4..a85c0119 100644
--- a/test/RpcMethodTest.cc
+++ b/test/RpcMethodTest.cc
@@ -915,7 +915,7 @@ void RpcMethodTest::testGatherStoppedDownload()
   d->fileEntries = fileEntries;
   d->inMemoryDownload = false;
   d->sessionDownloadLength = UINT64_MAX;
-  d->sessionTime = std::chrono::seconds(1);
+  d->sessionTime = 1_s;
   d->result = error_code::FINISHED;
   d->followedBy = followedBy;
   d->belongsTo = 2;
diff --git a/test/ServerStatManTest.cc b/test/ServerStatManTest.cc
index e87805e4..dd5bab15 100644
--- a/test/ServerStatManTest.cc
+++ b/test/ServerStatManTest.cc
@@ -159,7 +159,7 @@ void ServerStatManTest::testRemoveStaleServerStat()
   CPPUNIT_ASSERT(ssm.add(localhost_ftp));
   CPPUNIT_ASSERT(ssm.add(mirror));
 
-  ssm.removeStaleServerStat(std::chrono::hours(24));
+  ssm.removeStaleServerStat(24_h);
 
   CPPUNIT_ASSERT(ssm.find("localhost", "http"));
   CPPUNIT_ASSERT(!ssm.find("localhost", "ftp"));
diff --git a/test/TimeSeedCriteriaTest.cc b/test/TimeSeedCriteriaTest.cc
index aa751b26..2b6a124f 100644
--- a/test/TimeSeedCriteriaTest.cc
+++ b/test/TimeSeedCriteriaTest.cc
@@ -21,12 +21,12 @@ public:
 CPPUNIT_TEST_SUITE_REGISTRATION(TimeSeedCriteriaTest);
 
 void TimeSeedCriteriaTest::testEvaluate() {
-  TimeSeedCriteria cri(std::chrono::seconds(1));
+  TimeSeedCriteria cri(1_s);
   global::wallclock().reset();
-  global::wallclock().advance(std::chrono::seconds(2));
+  global::wallclock().advance(2_s);
   CPPUNIT_ASSERT(cri.evaluate());
   cri.reset();
-  cri.setDuration(std::chrono::seconds(10));
+  cri.setDuration(10_s);
   CPPUNIT_ASSERT(!cri.evaluate());
 }
 
diff --git a/test/UDPTrackerClientTest.cc b/test/UDPTrackerClientTest.cc
index f3604aef..423032a8 100644
--- a/test/UDPTrackerClientTest.cc
+++ b/test/UDPTrackerClientTest.cc
@@ -270,7 +270,7 @@ void UDPTrackerClientTest::testConnectFollowedByAnnounce()
   req4->infohash = "bittorrent-infohash4";
   tr.addRequest(req4);
   Timer future = now;
-  future.advance(std::chrono::hours(1));
+  future.advance(1_h);
   rv = tr.createRequest(data, sizeof(data), remoteAddr, remotePort,
                         future);
   // connection ID is stale because of the timeout
@@ -384,7 +384,7 @@ void UDPTrackerClientTest::testTimeout()
     CPPUNIT_ASSERT_EQUAL((int)UDPT_ACT_CONNECT,
                          (int)bittorrent::getIntParam(data, 8));
     tr.requestSent(now);
-    now.advance(std::chrono::seconds(20));
+    now.advance(20_s);
     // 15 seconds 1st stage timeout passed
     tr.handleTimeout(now);
     CPPUNIT_ASSERT(tr.getConnectRequests().empty());
@@ -396,7 +396,7 @@ void UDPTrackerClientTest::testTimeout()
     CPPUNIT_ASSERT_EQUAL((int)UDPT_ACT_CONNECT,
                          (int)bittorrent::getIntParam(data, 8));
     tr.requestSent(now);
-    now.advance(std::chrono::seconds(65));
+    now.advance(65_s);
     // 60 seconds 2nd stage timeout passed
     tr.handleTimeout(now);
     CPPUNIT_ASSERT(tr.getConnectRequests().empty());
@@ -428,7 +428,7 @@ void UDPTrackerClientTest::testTimeout()
     CPPUNIT_ASSERT_EQUAL((int)UDPT_ACT_ANNOUNCE,
                          (int)bittorrent::getIntParam(data, 8));
     tr.requestSent(now);
-    now.advance(std::chrono::seconds(20));
+    now.advance(20_s);
     // 15 seconds 1st stage timeout passed
     tr.handleTimeout(now);
     CPPUNIT_ASSERT(tr.getConnectRequests().empty());
@@ -439,7 +439,7 @@ void UDPTrackerClientTest::testTimeout()
     CPPUNIT_ASSERT_EQUAL((int)UDPT_ACT_ANNOUNCE,
                          (int)bittorrent::getIntParam(data, 8));
     tr.requestSent(now);
-    now.advance(std::chrono::seconds(65));
+    now.advance(65_s);
     // 60 seconds 2nd stage timeout passed
     tr.handleTimeout(now);
     CPPUNIT_ASSERT(tr.getConnectRequests().empty());
diff --git a/test/UTPexExtensionMessageTest.cc b/test/UTPexExtensionMessageTest.cc
index 9cf87690..cdb4910a 100644
--- a/test/UTPexExtensionMessageTest.cc
+++ b/test/UTPexExtensionMessageTest.cc
@@ -246,7 +246,7 @@ void UTPexExtensionMessageTest::testAddFreshPeer()
   std::shared_ptr<Peer> p1(new Peer("192.168.0.1", 6881));
   CPPUNIT_ASSERT(msg.addFreshPeer(p1));
   std::shared_ptr<Peer> p2(new Peer("10.1.1.2", 9999));
-  p2->setFirstContactTime(Timer(Timer().getTime() - std::chrono::seconds(61)));
+  p2->setFirstContactTime(Timer(Timer().getTime() - 61_s));
   CPPUNIT_ASSERT(!msg.addFreshPeer(p2));
   std::shared_ptr<Peer> p3(new Peer("10.1.1.3", 9999, true));
   CPPUNIT_ASSERT(!msg.addFreshPeer(p3));