[Android] system 시간 관련 함수

android   system 시간 관련 함수.

System.currentTimeMillis()
The wall clock can be set by the user or the phone
 *     network (see {@link#setCurrentTimeMillis}), 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.
 *     {@link android.content.Intent#ACTION_TIME_TICK ACTION_TIME_TICK},
 *     {@link android.content.Intent#ACTION_TIME_CHANGED ACTION_TIME_CHANGED}
 *     and {@link android.content.Intent#ACTION_TIMEZONE_CHANGED
 *     ACTION_TIMEZONE_CHANGED} {@link android.content.Intent Intent}
 *     broadcasts to find out when the time changes.
 *

System.uptimeMillis()
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.

System.elapsedRealtime() / System.elapsedRealtimeNanos()
 *     <li> <p> {@link#elapsedRealtime} and {@link#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.