## 13.6.3.4 MJD

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MJD (Modified Julian Day) is a scheme for counting days in a completely regular fashion. Each day is simply expressed by the number of days since a particular day.

MJD is a regular and reliable day count used by astronomers. Furthermore, it is politically correct in the sense that, even though Julius Caeser was quite controversial in his own time, few people today are offended by a time scale named after him.

MJD is based on yet another time scale named JD (Julian Day). JD expresses the number of days since noon, January 1, year -4712 (year 4713 BC), in the Julian calender.

In ancient times, a day was measured from noon to noon, so people actually counted nights instead of days (as a reminiscence, a period of 14 days is still called a fortnight in the English tongue). Today, we prefer to step our day counters when the sun is on the other side of the planet, which is of course difficult to observe, but which possesses little problem for modern technology.

To get a day count based on JD which steps at midnight, the Modified Julian Day (MJD) is offset from JD by 2400000.5 days. In consequence, MJD counts the number of days since GRD-1858-11-17.

When we combine MJD with UTC, then MJD steps at UTC:00:00:00. When we combine MJD with TAI, then MJD steps at TAI:00:00:00. Hence, MJD/UTC and MJD/TAI are two different day counts, but at the time of writing they merely differ by 34 seconds.

In Logiweb, MJD day d is written MJD-d. As an example, GRD-1858-11-17 equals MJD-0 and MJD-51544 equals GRD-2000-01-01. The day before MJD-0 is named MJD--1 (i.e. Modified Julian Day hyphen minus one). The notation follows ISO 8601 in using a hyphen in connection with day counting, but is otherwise completely unrelated.

Combinations of day and second counting schemes are glued together with a dot. As an example, 0.456 seconds past TAI:12:23:34 on MJD-51544 is written MJD-51544.TAI:12:23:34.456. This follows ISO-8601 in putting the day before the second but does not follow the suggestion of ISO-8601 to separate day and second by a capital 'T'.

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