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://www.slac.stanford.edu/~rkj/crazytime.txt
To make these dates fit in computer memory in the 1950s, they offset the calendar by 2.4 million days, placing day zero on November 17, 1858.