It’s called the modified Julian day (MJD). And the offset is 2,400,000.5 days. In the Julian day way of counting, each day ended at noon, so that all astronomic

It’s called the modified Julian day (MJD). And the offset is 2,400,000.5 days. In the Julian day way of counting, each day ended at noon, so that all astronomical observations done in one night would be the same Julian day, at least in Europe. MJD moved the epoch back to midnight. https://en.wikipedia.org/wiki/Julian_day https://en.wikipedia.org/wiki/Julian_day