Question

I want to calculate time difference between the last event on the device and the current. But the user can always change the time manually by setting the system time, so the difference can be different from the real time. Is there a way to calculate time difference that is correct?

Answer 1

You can use System.nanoTime() between the two periods, then subtract. Unlike System.currentTimeMillis() which changes when the user changes the device time, System.nanoTime() will not be affected.

Returns the current value of the most precise available system timer,
in nanoseconds.

This method can only be used to measure elapsed time and is not
related to any other notion of system or wall-clock time. The value
returned represents nanoseconds since some fixed but arbitrary time
(perhaps in the future, so values may be negative). This method
provides nanosecond precision, but not necessarily nanosecond
accuracy. No guarantees are made about how frequently values change.
Differences in successive calls that span greater than approximately
292 years (263 nanoseconds) will not accurately compute elapsed time
due to numerical overflow.