Observation Julian Dates

Astronomical community is sometimes using what are called "Julian dates". This has nothing to do with any form of calendar used in western countries however. Julian dates (JD) are a continuous count of days starting at January, 1st 4713 BC, Greenwich mean noon (12:00 UT). Such a count was devised in 1583 in Europe to make easier to compute dates intervals: result is always an integer number. Any day may be translated into a Julian date and has a structure like : 2452458.xxxx meaning that 2,452,458 days elapsed since day one of Julian dates (JD), as .xxxx accounts for clock time in the day. Any day may be converted into a JD date. It's enough to compute the difference between two dates and one has an integer interval between both. JD dates are from noon to noon UT

Modified Julian Date (MJD) is an additional system where 2,400,000.5 is substracted from the Julian date of the day. 2,400,000.5 is the number of days elapsed since November, 17th 1858, midnight (UTC). This system is yielding smaller and easier-to-use numbers

A Mars Sol Date (MSD) is sometimes used by Martian missions. MSD is a Martian Julian date system. Mars Sol Date is starting approximately at December, 29th 1873, noon UT. MSD is the number of Martian days elapsed from this starting point. "Sol" stands for "Martian day" as it is the other way days at Mars are termed. A "sol" is a "Martian day"