Absolute Time Alignment for ECEF and ECI

[ttarter3]

I have a project where it is assumed at time 0 seconds, the referenced equator and prime meridian for the ECI and ECEF frames are perfectly aligned. Pymap3d's eci2ecef uses an absolute time and the GRCS and ITRS frames seem to periodically be similar but on the order of 100s of meters different in the testing, I have performed. I wondered if there was a time when ECEF and ECI reference frames used by pympa3d were most similarly aligned, so I can make everything relative to that point.