Modified Julian Date (S01 E00)

What is MJD 60787, and why do astronomers, GPS engineers, and half the internet use it instead of a calendar? Modified Julian Date is a single running counter of days since midnight UT on 17 November 1858. No months, no time zones, no leap days; one monotone real number that happens to be a Unix timestamp with a constant subtracted off. S1E0 is a 20-minute tour: what MJD is, why 1858, how to convert it in three one-liners, where you've already been seeing it (TLE, RINEX, FITS, BIPM Circular T), and how leap seconds interact with UTC, TAI, and GPST. Hands-on companion site: https://modifiedjulian.date