Fix parsing botch when ending a ramp

When ending a ramp, the caller (sps) passes in the real obstime from the shutters. This value is then used to recalculate all the FITS time cards and those cards in the PHDU are rewritten from the pre-exposure guesstimate.

A recent change in the command parsing broke this, and the "current time" (the end of the ramp, actually) was used instead. Fix.