android Difference between handler.postAtTime and handler.postDelayed

Please tell me the difference between handler.postAtTime and handler.postDelayed in android.And also please guide me when to use handler.postAtTime and when to use 开发者_JS百科handler.postDelayed.

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From documentation:

For postAtTime:

public final boolean postAtTime (Runnable r, long uptimeMillis)
...
uptimeMillis The absolute time at which the callback should run, using the uptimeMillis() time-base. ...

And for postDelayed:

public final boolean postDelayed (Runnable r, long delayMillis)
...
delayMillis The delay (in milliseconds) until the Runnable will be executed. ...

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If this is still not clear, postDelayed() run something after X millisecond from current time. While postAtTime() runs something at specified time XX:YY:ZZ.mmm.