The Gregorian Leap Year
In general every Gregorian Calendar year divisible by 4 is a leap year, except if it is divisible by 100 but not divisible by 400.
The Gregorian Calendar was adopted in 1582 by Rome and by other countries between 1582 to 1924. Britain and possessions however, only changed from the Julian to the Gregorian Calendar in 1752.
“Therefore, RC & GB used the same Julian rules before 1600; different rules but with the same result in 1600; different rules with different results in 1700; and the same Gregorian rules thereafter. Only 1700 differed in Leapness between RC & GB; the historical Year 1700 was a Leap Year in Britain and possessions. Note that many otherwise reputable authorities have omitted to consider this point; but the differing Leapness of Historical 1700 is what is needed to reconcile the 10- and 11- day jumps of 1582 and 1752*. Therefore, the Gregorian date 29th February 1700 occurred here in GB, but on the civil date 29th February 1699 (the new year number then starting with March 25th).
The Thursday which was Julian (UK) 1700-02-29 was MJD -57959 and Gregorian (Rome) 1700-03-11. “
*Great Britain and Dominions: September 2 1752 was followed by September 14 1752.Eleven days rather than the 10 days when the Gregorian calendar was adopted in Rome (October 4 1582 was followed by October 15 1582)