System.currentTimeMillis measuring time

When I use System.currentTimeMillis() to measure the time, is it safe?

E.g. When a time-shift happens (summer->winter time) will this produce an error?

Jon Skeet
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No, System.currentTimeMillis() returns the number of elapsed milliseconds since the Unix epoch (midnight at the start of January 1st 1970, UTC). This does not depend on your local time zone at all.

In other words, if you call it once per second, it will always increase by about 1000 (the "about" is only due to clock accuracy and the impossibility of the calls being exactly 1 second apart). It doesn't matter if your local time zone changes its offset from UTC, or if you even change your whole time zone - that will be irrelevant.

In theory, leap seconds make all of this trickier - but in practice most applications can get away with notionally sticking their fingers in their ears and saying "I'm not at home to Mr Leap Second."

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