MJD is the Modified Julian Date. MJD = JD - 2400000.5 . (Note that it starts from midnight rather than noon.)

JD is the Julian Date. JD = 0 at 12:00 January 1, 4713 BCE, Monday.

Note: Both JD and MJD have associated time scales. If no qualifier is present, then the time scale is assumed to be TT, as in MJD(TT). However, MJD(UTC), for example, indicates that the epoch is given in Coordinated Universal Time (UTC). Normally, a UTC day is 86400 SI or atmoic seconds; except for those days in which a leap second occurs. In that case, it is 86401 SI seconds in length. TT days are always 86400 SI seconds long.