Add aliases for chrono facilities.

diff --git a/asio/include/asio/detail/chrono.hpp b/asio/include/asio/detail/chrono.hpp
new file mode 100644
index 0000000..93cf894
--- /dev/null
+++ b/asio/include/asio/detail/chrono.hpp

@@ -0,0 +1,66 @@
+//
+// detail/chrono.hpp
+// ~~~~~~~~~~~~~~~~~
+//
+// Copyright (c) 2003-2015 Christopher M. Kohlhoff (chris at kohlhoff dot com)
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+
+#ifndef ASIO_DETAIL_CHRONO_HPP
+#define ASIO_DETAIL_CHRONO_HPP
+
+#if defined(_MSC_VER) && (_MSC_VER >= 1200)
+# pragma once
+#endif // defined(_MSC_VER) && (_MSC_VER >= 1200)
+
+#include "asio/detail/config.hpp"
+
+#if defined(ASIO_HAS_STD_CHRONO)
+# include <chrono>
+#elif defined(ASIO_HAS_BOOST_CHRONO)
+# include <boost/chrono/system_clocks.hpp>
+#endif // defined(ASIO_HAS_BOOST_CHRONO)
+
+namespace asio {
+namespace chrono {
+
+#if defined(ASIO_HAS_STD_CHRONO)
+using std::chrono::duration;
+using std::chrono::time_point;
+using std::chrono::duration_cast;
+using std::chrono::nanoseconds;
+using std::chrono::microseconds;
+using std::chrono::milliseconds;
+using std::chrono::seconds;
+using std::chrono::minutes;
+using std::chrono::hours;
+using std::chrono::time_point_cast;
+#if defined(ASIO_HAS_STD_CHRONO_MONOTONIC_CLOCK)
+typedef std::chrono::monotonic_clock system_clock;
+#else // defined(ASIO_HAS_STD_CHRONO_MONOTONIC_CLOCK)
+using std::chrono::system_clock;
+#endif // defined(ASIO_HAS_STD_CHRONO_MONOTONIC_CLOCK)
+using std::chrono::steady_clock;
+using std::chrono::high_resolution_clock;
+#elif defined(ASIO_HAS_BOOST_CHRONO)
+using boost::chrono::duration;
+using boost::chrono::time_point;
+using boost::chrono::duration_cast;
+using boost::chrono::nanoseconds;
+using boost::chrono::microseconds;
+using boost::chrono::milliseconds;
+using boost::chrono::seconds;
+using boost::chrono::minutes;
+using boost::chrono::hours;
+using boost::chrono::time_point_cast;
+using boost::chrono::system_clock;
+using boost::chrono::steady_clock;
+using boost::chrono::high_resolution_clock;
+#endif // defined(ASIO_HAS_BOOST_CHRONO)
+
+} // namespace chrono
+} // namespace asio
+
+#endif // ASIO_DETAIL_CHRONO_HPP

diff --git a/asio/include/asio/detail/config.hpp b/asio/include/asio/detail/config.hpp
index 43a1e1c..01e499f 100644
--- a/asio/include/asio/detail/config.hpp
+++ b/asio/include/asio/detail/config.hpp

@@ -454,6 +454,15 @@
 # endif // !defined(ASIO_DISABLE_BOOST_CHRONO)
 #endif // !defined(ASIO_HAS_BOOST_CHRONO)
 
+// Some form of chrono library is available.
+#if !defined(ASIO_HAS_CHRONO)
+# if defined(ASIO_HAS_STD_CHRONO) \
+    || defined(ASIO_HAS_BOOST_CHRONO)
+#  define ASIO_HAS_CHRONO 1
+# endif // defined(ASIO_HAS_STD_CHRONO)
+        // || defined(ASIO_HAS_BOOST_CHRONO)
+#endif // !defined(ASIO_HAS_CHRONO)
+
 // Boost support for the DateTime library.
 #if !defined(ASIO_HAS_BOOST_DATE_TIME)
 # if !defined(ASIO_DISABLE_BOOST_DATE_TIME)

diff --git a/asio/include/asio/detail/impl/handler_tracking.ipp b/asio/include/asio/detail/impl/handler_tracking.ipp
index e838a93..4095a01 100644
--- a/asio/include/asio/detail/impl/handler_tracking.ipp
+++ b/asio/include/asio/detail/impl/handler_tracking.ipp

@@ -29,12 +29,8 @@
 
 #if defined(ASIO_HAS_BOOST_DATE_TIME)
 # include "asio/time_traits.hpp"
-#else // defined(ASIO_HAS_BOOST_DATE_TIME)
-# if defined(ASIO_HAS_STD_CHRONO)
-#  include <chrono>
-# elif defined(ASIO_HAS_BOOST_CHRONO)
-#  include <boost/chrono/system_clocks.hpp>
-# endif
+#elif defined(ASIO_HAS_CHRONO)
+# include "asio/detail/chrono.hpp"
 # include "asio/detail/chrono_time_traits.hpp"
 # include "asio/wait_traits.hpp"
 #endif // defined(ASIO_HAS_BOOST_DATE_TIME)
@@ -61,16 +57,11 @@
     boost::posix_time::ptime epoch(boost::gregorian::date(1970, 1, 1));
     boost::posix_time::time_duration now =
       boost::posix_time::microsec_clock::universal_time() - epoch;
-#elif defined(ASIO_HAS_STD_CHRONO)
-    typedef chrono_time_traits<std::chrono::system_clock,
-        asio::wait_traits<std::chrono::system_clock> > traits_helper;
+#elif defined(ASIO_HAS_CHRONO)
+    typedef chrono_time_traits<chrono::system_clock,
+        asio::wait_traits<chrono::system_clock> > traits_helper;
     traits_helper::posix_time_duration now(
-        std::chrono::system_clock::now().time_since_epoch());
-#elif defined(ASIO_HAS_BOOST_CHRONO)
-    typedef chrono_time_traits<boost::chrono::system_clock,
-        asio::wait_traits<boost::chrono::system_clock> > traits_helper;
-    traits_helper::posix_time_duration now(
-        boost::chrono::system_clock::now().time_since_epoch());
+        chrono::system_clock::now().time_since_epoch());
 #endif
     seconds = static_cast<uint64_t>(now.total_seconds());
     microseconds = static_cast<uint64_t>(now.total_microseconds() % 1000000);

diff --git a/asio/include/asio/high_resolution_timer.hpp b/asio/include/asio/high_resolution_timer.hpp
index 6ab1324..133d292 100644
--- a/asio/include/asio/high_resolution_timer.hpp
+++ b/asio/include/asio/high_resolution_timer.hpp

@@ -17,21 +17,13 @@
 
 #include "asio/detail/config.hpp"
 
-#if defined(ASIO_HAS_STD_CHRONO) \
-  || defined(ASIO_HAS_BOOST_CHRONO) \
-  || defined(GENERATING_DOCUMENTATION)
-
-#if defined(ASIO_HAS_STD_CHRONO)
-# include <chrono>
-#elif defined(ASIO_HAS_BOOST_CHRONO)
-# include <boost/chrono/system_clocks.hpp>
-#endif
+#if defined(ASIO_HAS_CHRONO) || defined(GENERATING_DOCUMENTATION)
 
 #include "asio/basic_waitable_timer.hpp"
+#include "asio/detail/chrono.hpp"
 
 namespace asio {
 
-#if defined(GENERATING_DOCUMENTATION)
 /// Typedef for a timer based on the high resolution clock.
 /**
  * This typedef uses the C++11 @c &lt;chrono&gt; standard library facility, if
@@ -44,20 +36,9 @@
 typedef basic_waitable_timer<
     chrono::high_resolution_clock>
   high_resolution_timer;
-#elif defined(ASIO_HAS_STD_CHRONO)
-typedef basic_waitable_timer<
-    std::chrono::high_resolution_clock>
-  high_resolution_timer;
-#elif defined(ASIO_HAS_BOOST_CHRONO)
-typedef basic_waitable_timer<
-    boost::chrono::high_resolution_clock>
-  high_resolution_timer;
-#endif
 
 } // namespace asio
 
-#endif // defined(ASIO_HAS_STD_CHRONO) 
-       //   || defined(ASIO_HAS_BOOST_CHRONO)
-       //   || defined(GENERATING_DOCUMENTATION)
+#endif // defined(ASIO_HAS_CHRONO) || defined(GENERATING_DOCUMENTATION)
 
 #endif // ASIO_HIGH_RESOLUTION_TIMER_HPP

diff --git a/asio/include/asio/steady_timer.hpp b/asio/include/asio/steady_timer.hpp
index 8e7ca68..73990a9 100644
--- a/asio/include/asio/steady_timer.hpp
+++ b/asio/include/asio/steady_timer.hpp

@@ -17,21 +17,13 @@
 
 #include "asio/detail/config.hpp"
 
-#if defined(ASIO_HAS_STD_CHRONO) \
-  || defined(ASIO_HAS_BOOST_CHRONO) \
-  || defined(GENERATING_DOCUMENTATION)
-
-#if defined(ASIO_HAS_STD_CHRONO)
-# include <chrono>
-#elif defined(ASIO_HAS_BOOST_CHRONO)
-# include <boost/chrono/system_clocks.hpp>
-#endif
+#if defined(ASIO_HAS_CHRONO) || defined(GENERATING_DOCUMENTATION)
 
 #include "asio/basic_waitable_timer.hpp"
+#include "asio/detail/chrono.hpp"
 
 namespace asio {
 
-#if defined(GENERATING_DOCUMENTATION)
 /// Typedef for a timer based on the steady clock.
 /**
  * This typedef uses the C++11 @c &lt;chrono&gt; standard library facility, if
@@ -42,20 +34,9 @@
  * @endcode
  */
 typedef basic_waitable_timer<chrono::steady_clock> steady_timer;
-#elif defined(ASIO_HAS_STD_CHRONO)
-# if defined(ASIO_HAS_STD_CHRONO_MONOTONIC_CLOCK)
-typedef basic_waitable_timer<std::chrono::monotonic_clock> steady_timer;
-# else // defined(ASIO_HAS_STD_CHRONO_MONOTONIC_CLOCK)
-typedef basic_waitable_timer<std::chrono::steady_clock> steady_timer;
-# endif // defined(ASIO_HAS_STD_CHRONO_MONOTONIC_CLOCK)
-#elif defined(ASIO_HAS_BOOST_CHRONO)
-typedef basic_waitable_timer<boost::chrono::steady_clock> steady_timer;
-#endif
 
 } // namespace asio
 
-#endif // defined(ASIO_HAS_STD_CHRONO) 
-       //   || defined(ASIO_HAS_BOOST_CHRONO)
-       //   || defined(GENERATING_DOCUMENTATION)
+#endif // defined(ASIO_HAS_CHRONO) || defined(GENERATING_DOCUMENTATION)
 
 #endif // ASIO_STEADY_TIMER_HPP

diff --git a/asio/include/asio/system_timer.hpp b/asio/include/asio/system_timer.hpp
index 143056e..8476ac0 100644
--- a/asio/include/asio/system_timer.hpp
+++ b/asio/include/asio/system_timer.hpp

@@ -17,21 +17,13 @@
 
 #include "asio/detail/config.hpp"
 
-#if defined(ASIO_HAS_STD_CHRONO) \
-  || defined(ASIO_HAS_BOOST_CHRONO) \
-  || defined(GENERATING_DOCUMENTATION)
-
-#if defined(ASIO_HAS_STD_CHRONO)
-# include <chrono>
-#elif defined(ASIO_HAS_BOOST_CHRONO)
-# include <boost/chrono/system_clocks.hpp>
-#endif
+#if defined(ASIO_HAS_CHRONO) || defined(GENERATING_DOCUMENTATION)
 
 #include "asio/basic_waitable_timer.hpp"
+#include "asio/detail/chrono.hpp"
 
 namespace asio {
 
-#if defined(GENERATING_DOCUMENTATION)
 /// Typedef for a timer based on the system clock.
 /**
  * This typedef uses the C++11 @c &lt;chrono&gt; standard library facility, if
@@ -42,16 +34,9 @@
  * @endcode
  */
 typedef basic_waitable_timer<chrono::system_clock> system_timer;
-#elif defined(ASIO_HAS_STD_CHRONO)
-typedef basic_waitable_timer<std::chrono::system_clock> system_timer;
-#elif defined(ASIO_HAS_BOOST_CHRONO)
-typedef basic_waitable_timer<boost::chrono::system_clock> system_timer;
-#endif
 
 } // namespace asio
 
-#endif // defined(ASIO_HAS_STD_CHRONO) 
-       //   || defined(ASIO_HAS_BOOST_CHRONO)
-       //   || defined(GENERATING_DOCUMENTATION)
+#endif // defined(ASIO_HAS_CHRONO) || defined(GENERATING_DOCUMENTATION)
 
 #endif // ASIO_SYSTEM_TIMER_HPP

diff --git a/asio/src/tests/unit/io_context.cpp b/asio/src/tests/unit/io_context.cpp
index 833c1b7..4617547 100644
--- a/asio/src/tests/unit/io_context.cpp
+++ b/asio/src/tests/unit/io_context.cpp

@@ -43,12 +43,9 @@
 #if defined(ASIO_HAS_BOOST_DATE_TIME)
 typedef deadline_timer timer;
 namespace chronons = boost::posix_time;
-#elif defined(ASIO_HAS_STD_CHRONO)
+#elif defined(ASIO_HAS_CHRONO)
 typedef steady_timer timer;
-namespace chronons = std::chrono;
-#elif defined(ASIO_HAS_BOOST_CHRONO)
-typedef steady_timer timer;
-namespace chronons = boost::chrono;
+namespace chronons = asio::chrono;
 #endif // defined(ASIO_HAS_BOOST_DATE_TIME)
 
 void increment(int* count)

diff --git a/asio/src/tests/unit/strand.cpp b/asio/src/tests/unit/strand.cpp
index 5b01e2d..3db5021 100644
--- a/asio/src/tests/unit/strand.cpp
+++ b/asio/src/tests/unit/strand.cpp

@@ -45,12 +45,9 @@
 #if defined(ASIO_HAS_BOOST_DATE_TIME)
 typedef deadline_timer timer;
 namespace chronons = boost::posix_time;
-#elif defined(ASIO_HAS_STD_CHRONO)
+#elif defined(ASIO_HAS_CHRONO)
 typedef steady_timer timer;
-namespace chronons = std::chrono;
-#elif defined(ASIO_HAS_BOOST_CHRONO)
-typedef steady_timer timer;
-namespace chronons = boost::chrono;
+namespace chronons = asio::chrono;
 #endif // defined(ASIO_HAS_BOOST_DATE_TIME)
 
 void increment(int* count)

diff --git a/asio/src/tests/unit/system_timer.cpp b/asio/src/tests/unit/system_timer.cpp
index 9aa41ce..9d382ee 100644
--- a/asio/src/tests/unit/system_timer.cpp
+++ b/asio/src/tests/unit/system_timer.cpp

@@ -40,8 +40,6 @@
 namespace bindns = std;
 #endif // defined(ASIO_HAS_BOOST_BIND)
 
-namespace chronons = std::chrono;
-
 void increment(int* count)
 {
   ++(*count);
@@ -55,7 +53,7 @@
 
     int before_value = *count;
 
-    t->expires_at(t->expiry() + chronons::seconds(1));
+    t->expires_at(t->expiry() + asio::chrono::seconds(1));
     t->async_wait(bindns::bind(decrement_to_zero, t, count));
 
     // Completion cannot nest, so count value should remain unchanged.
@@ -89,8 +87,8 @@
 
 void system_timer_test()
 {
-  using chronons::seconds;
-  using chronons::microseconds;
+  using asio::chrono::seconds;
+  using asio::chrono::microseconds;
 #if !defined(ASIO_HAS_BOOST_BIND)
   using std::placeholders::_1;
   using std::placeholders::_2;
@@ -334,13 +332,13 @@
 
   asio::thread th(bindns::bind(io_context_run, &ioc));
 
-  t2.expires_after(chronons::seconds(2));
+  t2.expires_after(asio::chrono::seconds(2));
   t2.wait();
 
-  t1.expires_after(chronons::seconds(2));
+  t1.expires_after(asio::chrono::seconds(2));
   t1.async_wait(bindns::bind(increment, &count));
 
-  t2.expires_after(chronons::seconds(4));
+  t2.expires_after(asio::chrono::seconds(4));
   t2.wait();
 
   ioc.stop();
@@ -353,7 +351,7 @@
 asio::system_timer make_timer(asio::io_context& ioc, int* count)
 {
   asio::system_timer t(ioc);
-  t.expires_after(std::chrono::seconds(1));
+  t.expires_after(asio::chrono::seconds(1));
   t.async_wait(bindns::bind(increment, count));
   return t;
 }