Core timekeeping facilities.

Three different clocks are available, and they should not be confused:

• Java.Lang.JavaSystem.CurrentTimeMillis is the standard "wall" clock (time and date) expressing milliseconds since the epoch. The wall clock can be set by the user or the phone network (see SystemClock.SetCurrentTimeMillis(long)), so the time may jump backwards or forwards unpredictably. This clock should only be used when correspondence with real-world dates and times is important, such as in a calendar or alarm clock application. Interval or elapsed time measurements should use a different clock. If you are using System.currentTimeMillis(), consider listening to the Android.Content.Intent.ActionTimeTick, Android.Content.Intent.ActionTimeChanged and Android.Content.Intent.ActionTimezoneChangedAndroid.Content.Intent broadcasts to find out when the time changes.
• SystemClock.UptimeMillis is counted in milliseconds since the system was booted. This clock stops when the system enters deep sleep (CPU off, display dark, device waiting for external input), but is not affected by clock scaling, idle, or other power saving mechanisms. This is the basis for most interval timing such as Java.Lang.Thread.Sleep(long), Java.Lang.Object.Wait(long), and Java.Lang.JavaSystem.NanoTime. This clock is guaranteed to be monotonic, and is suitable for interval timing when the interval does not span device sleep. Most methods that accept a timestamp value currently expect the SystemClock.UptimeMillis clock.
• SystemClock.ElapsedRealtime and SystemClock.ElapsedRealtimeNanos return the time since the system was booted, and include deep sleep. This clock is guaranteed to be monotonic, and continues to tick even when the CPU is in power saving modes, so is the recommend basis for general purpose interval timing.

• Standard functions like Java.Lang.Thread.Sleep(long) and Java.Lang.Object.Wait(long) are always available. These functions use the SystemClock.UptimeMillis clock; if the device enters sleep, the remainder of the time will be postponed until the device wakes up. These synchronous functions may be interrupted with Java.Lang.Thread.Interrupt, and you must handle Java.Lang.InterruptedException.
• SystemClock.Sleep(long) is a utility function very similar to Java.Lang.Thread.Sleep(long), but it ignores Java.Lang.InterruptedException. Use this function for delays if you do not use Java.Lang.Thread.Interrupt, as it will preserve the interrupted state of the thread.
• The Android.OS.Handler class can schedule asynchronous callbacks at an absolute or relative time. Handler objects also use the SystemClock.UptimeMillis clock, and require an Android.OS.Looper (normally present in any GUI application).
• The Android.App.AlarmManager can trigger one-time or recurring events which occur even when the device is in deep sleep or your application is not running. Events may be scheduled with your choice of Java.Lang.JavaSystem.CurrentTimeMillis (RTC) or SystemClock.ElapsedRealtime (ELAPSED_REALTIME), and cause an Android.Content.Intent broadcast when they occur.

[Android Documentation]