A moment in time used as a reference point for some time-varying astronomical quantity.
The most popular ones are (TfT means terrestrial time)
- Julian Days -> JD 2433282.4235 for January 0.9235, 1950 TT
- besselian-year -> 1950.0 or B1950.0 for January 0.9235, 1950 TT
- Julian Years -> J2000.0 for January 1.5, 2000 TT