public final class SystemClock extends Object

Core timekeeping facilities.

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

  • System.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 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.

  • 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 Thread.sleep(millls), Object.wait(millis), and System.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 uptimeMillis() clock.

  • elapsedRealtime() and 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.

There are several mechanisms for controlling the timing of events:

Public Method Summary

static long
Returns milliseconds since boot, including time spent in sleep.
static long
Returns nanoseconds since boot, including time spent in sleep.
static boolean
setCurrentTimeMillis(long millis)
Sets the current wall time, in milliseconds.
static void
sleep(long ms)
Waits a given number of milliseconds (of uptimeMillis) before returning.