The function System.currentTimeMillis() returns the number of milliseconds since January 1, 1970 00:00:00 UTC.
The function Date.getTime() returns the number of milliseconds since January 1, 1970 GMT midnight.
This is what Android’s documentation says.
So where is the difference? Can I calculate with them or is there an offset?
For most intents and purposes the two are equivalent.
http://en.wikipedia.org/wiki/Coordinated_Universal_Time#Definition_and_relationship_to_other_standards