a system clock from simgear, using std::chrono libs

this is to use some kind of utc time for timestamps in mp protocol,
to improve the lag correction system for mp planes, using a
"real time" mode when possible.

simgear/timing/timestamp.cxx

@@ -29,6 +29,7 @@
 
 #include <ctime>
 #include <cerrno>
+#include <chrono>
 
 #ifdef HAVE_UNISTD_H
 #  include <unistd.h>    // for gettimeofday() and the _POSIX_TIMERS define
@@ -98,6 +99,21 @@ void SGTimeStamp::stamp()
 #endif
 }
 
+void SGTimeStamp::systemClockHoursAndMinutes()
+{
+    using namespace std;
+    using namespace std::chrono;
+
+    typedef duration<int, ratio_multiply<hours::period, ratio<24> >::type> days;
+
+    system_clock::time_point now = system_clock::now();
+    system_clock::duration tp = now.time_since_epoch();
+    tp -= duration_cast<days>(tp);
+
+    _sec = duration_cast<seconds>(tp).count();
+    _nsec = duration_cast<nanoseconds>(tp - seconds(_sec)).count();
+}
+
 // sleep based timing loop.
 //
 // Calling sleep, even usleep() on linux is less accurate than

simgear/timing/timestamp.hxx

@@ -78,6 +78,13 @@ public:
     /** Update stored time to current time (seconds and nanoseconds) */
     void stamp();
 
+    /** Update stored time to current system clock (seconds and nanoseconds)
+     *  non monotonic, keeping only hours, minutes, seconds and decimals,
+     *  so restart at 0 at midnight.
+     *  using the std::chrono libs
+     */
+    void systemClockHoursAndMinutes();
+
     /** Set the time from a double value */
     void setTime(const double& seconds)
     {