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 (t_o) 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