Since it's a long it is probably the system time in milliseconds:
public static long currentTimeMillis()
Returns the current time in milliseconds. Note that while the unit of time of the return value is a millisecond, the granularity of the value depends on the underlying operating system and may be larger. For example, many operating systems measure time in units of tens of milliseconds.
See the description of the class Date for a discussion of slight discrepancies that may arise between "computer time" and coordinated universal time (UTC).
Returns: the difference, measured in milliseconds, between the current time and midnight, January 1, 1970 UTC.