Fun fact: astronomers (and Nasa) use the Julian calendar for events (for example eclipses) before october 1582. You'd think they use UT, but they don't, so you can match historical dates with celestial events. Also worth noting, astronomical year numbering has a year 0, whereas BC/AD system does not, so that year 0 is 1 BC.

You have to make time corrections when you program something related to astronomy. In most things we program, UT is an absolute on which we can rely, but when we go very far away in the past it's not that clear anymore.