The 'modern' calendar is based upon the amount of time it takes the Earth to go through one seasonal cycle (called a 'Tropical Year' - about 365.25 days). More than 2,000 years ago, the Romans were aware of this and made every fourth year have 366 days which was accurate enough for a timespan of a few centuries. The Romans (and earlier the Greeks) were even aware that the tropical year was slightly less than 365.25 days but no provision was made to adjust the calendar for this very slight correction. So, by the 1500's, the calendar was 10 days out of step with the seasons. Most of Europe adjusted their calendars by eliminating 10 days of the calendar in 1582.

Other countries eventually compensated for this and a new calendar rule was adopted. Every 4th year would still be a Leap Year with special exceptions for certain "Century Years" (1700, 1800, 1900, etc). If such a year is divided by 400 and has no remainder, then it will be a leap year as is the case with 2000 and 2400. If there is a remainder, then it will only be a 365 day year as is the case with 1700, 1800, 1900, 2100, etc. You may test this by inputting February for these years.

Are you wondering what JD, MJD and TJD are?
JD stands for Julian Day Number which had its starting point at NOON January 1, 4713 BC, making NOON January 2, 4713 BC Julian Day Number 1. Millions of days have passed since then and as of NOON January 1, 2000 the Julian Day Number was 2,451,545. Astronomers use Julian Day Numbers because of the immense time span covered and because each 'day' begins at noon. Since the vast majority of astronomical events and activities occur at night, it is very convenient to have one night's observations not split between 2 days. Also, the Julian Day Number can conveniently incorporate the time as well. For example, January 1, 2000 at 3:00 a.m. is Julian Day Number 2,451,544.625. Well, that's all well and good for astronomy, but rather awkward for everything else. After all, having a day expressed as a 7 digit integer that changes at noontime does not fit in well with most human activities. So, a slightly different JD Numbering system was conceived called: