In order to predict the future position of a satellite, you must first indicate the time that you wish to observe it.
Let's set this time to 00:19:51.670 UTC December 18, 2007, which corresponds to a routine CASTOR image taken of the GlobalStar M047 satellite:
t = 00:19:51.670 UTC December 18, 2007
The propagation time (Dt) is the time elapsed from the Epoch time (to) to our chosen time (t). In other words, Dt is the difference between the two times (t - t0):
Propagation Time = Dt = t - t0
Dt = 1.7677141 solar days
Step 2: Selecting Propagation Time Was Last Modified On January 03, 2012