AK: Introduce UnixDateTime

This is a generic wrapper for a time instant relative to the unix epoch,
and does not account for leap seconds. It should be used in place of
Duration in most current cases.

AK/Time.cpp

@@ -237,6 +237,16 @@ Duration Duration::now_monotonic_coarse()
     return now_time_from_clock(CLOCK_MONOTONIC_COARSE);
 }
 
+UnixDateTime UnixDateTime::now()
+{
+    return UnixDateTime { now_time_from_clock(CLOCK_REALTIME) };
+}
+
+UnixDateTime UnixDateTime::now_coarse()
+{
+    return UnixDateTime { now_time_from_clock(CLOCK_REALTIME_COARSE) };
+}
+
 #endif
 
 }