The Gregorian calendar today serves as an international standard for civil use. Years are counted from the initial epoch defined by Dionysius Exiguus, and are divided into two classes: common years and leap years.

A common year is 365 days in length;

A leap year is 366 days, with an intercalary day, designated February 29, preceding March 1.

Leap years are determined according to the following rule:

Every year that is exactly divisible by 4 is a leap year, except for years that are exactly divisible by 100; these centurial years are leap years only if they are exactly divisible by 400.

The Julian day number of a day is the number of days (or part of a day) elapsed since noon GMT (or more exactly, UT) on January 1st, 4713 B.C., in the Proleptic Julian Calendar.

Modified Julian Day is defined as the Julian Day minus 2400000.5.

Thus MJD 0 is at midnight between the 16 and 17 November 1858 AD Gregorian.

LOP Julian day is defined as Modified Julian Day minus 48622.

Thus LOP 0 is at midnight between the 31 December and 01 January 1992 AD Gregorian.

This is the origin chosen by our team at Laboratoire d'Oceanographie Physique, Museum National d'Histoire Naturelle, PARIS, FRANCE.

CNES Julian day is defined as Modified Julian Day minus 33282.

Thus CNES 0 is at midnight between the 31 December and 01 January 1950 AD Gregorian.

This is the format used in CNES products like MSLA data files.

NASA Julian day is defined as Modified Julian Day minus 40000.

Thus NASA 0 is at midnight between the 23 May and 24 May 1968 AD Gregorian, at which time the Apollo missions to the Moon were underway.

CCSDS Julian day is defined as Modified Julian Day minus 36204.

Thus CCSDS 0 is at midnight between the 31 December and 01 January 1958 AD Gregorian.

This is the CONSULTATIVE COMMITTEE FOR SPACE DATA SYSTEMS recommended epoch, and this is the one used in TOPEX/POSEIDON altimetry.

On 10 August 1992, the joint US/French altimetric satellite TOPEX/POSEIDON was launched on an Ariane rocket from Kourou, French Guyana.

The satellite was maneuvered to an orbit with a ten-day (a CYCLE) repeat ground track of 127 revolutions.

One revolution is segmented in a descending PASS and an ascending PASS, i.e. 254 PASS.

TPtime TOPEX CYCLE date corresponds to the beginning of this CYCLE, at the beginning of the displayed PASS.

For calculation simplification, we assume that the duration of a PASS is always equal to 3372.87973 s, that is not always exactly true. So there could be some differences between the computed date and the real one, provided by NASA or CNES.

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