[logfromblammo]

For doing a ballistic propagation, you apply a gravitational map in Earth-centered, Earth-fixed (ECEF) geodetic coordinates, then convert to Earth-centered rotating (ECR) geodetic coordinates, because that way you don't have to correct for the Coriolis effect. That ECEF-ECR conversion requires a time-of-day parameter.

You can use a gravitational map that only accounts for latitude, but it isn't as precise.

So using an accurate clock is really important if your intent is to hit a missile with a missile.