Argh time is annoying, if you care about it at the sub-second level! Lang and I spent a chunk of the day converting from GALEX spacecraft clock time (which is more-or-less synchronized to GPS time) to barycentric time (which is the time you would be keeping if you were a fictional Newtonian observer at the Solar System Barycenter). This is necessary because we care about eclipse timing at the sub-second level, and the light-travel time from Earth to the Barycenter is minutes. The light-travel time from the center of the Sun (Heliocentric frame) to the Barycenter is seconds. We spent a bit of our time trying to robotically exercise the JPL Horizons ephemeris, which was successful but challenging.