Calendars

The ordinary Gregorian Calendar Date, together with a time of day, can be used to express an epoch in any desired timescale. For many purposes, however, a continuous count of days is more convenient, and for this purpose the system of Julian Day Number can be used. JD zero is located about 7000 years ago, well before the historical era, and is formally defined in terms of Greenwich noon; for example Julian Day Number 2449444 began at noon on 1994 April 1. Julian Date is the same system but with a fractional part appended; Julian Date 2449443.5 was the midnight on which 1994 April 1 commenced. Because of the unwieldy size of Julian Dates and the awkwardness of the half-day offset, it is accepted practice to remove the leading `24' and the trailing `.5', producing what is called the Modified Julian Date: MJD = JD-2400000.5. SLALIB routines use MJD, as opposed to JD, throughout, largely to avoid loss of precision. 1994 April 1 commenced at MJD 49443.0.

Despite JD (and hence MJD) being defined in terms of (in effect) UT, the system can be used in conjunction with other timescales such as TAI, TT and TDB (and even sidereal time through the concept of Greenwich Sidereal Date). However, it is improper to express a UTC as a JD or MJD because of leap seconds.

SLALIB has six routines for converting to and from dates in the Gregorian calendar. The routines sla_CLDJ and sla_CALDJ both convert a calendar date into an MJD, the former interpreting years between 0 and 99 as 1st century and the latter as late 20th or early 21st century. The routines sla_DJCL and sla_DJCAL both convert an MJD into calendar year, month, day and fraction of a day; the latter performs rounding to a specified precision, important to avoid dates like `94 04 01.***' appearing in messages. Some of SLALIB's low-precision ephemeris routines (sla_EARTH, sla_MOON and sla_ECOR) work in terms of year plus day-in-year (where day 1 = January 1st, at least for the modern era). This form of date can be generated by calling sla_CALYD (which defaults years 0-99 into 1950-2049) or sla_CLYD (which covers the full range from prehistoric times).