Are you looking to generate something like a unix timestamp for the data? Or to find a timestamp that wont collide with an existing file? If its the later you could always use ticks.
The problem with any "long" timestamp is that it will be relative to the machine generating it but wont guarantee non-collision on the other system, as the clocks can be set differently (not float, but actually be set differently).
If the data is secure/sensitive and you are looking at a time-based mechanism for sync-ing keys (ALA Kerberos) I would not suggest rolling your own as there are many obstacles to overcome especially in sync-ing systems and keeping them in sync.