What is the modified Julian date?

Modified Julian Day (MJD) was defined in the mid 1950s in the interests of astronomy and space science as MJD = JD - 2400000.5, which accomplishes two things. The half day shift makes the day start at midnight, which is the current time standard.

What are the units of Julian date?

The months January to December are numbered 1 to 12. For the year, astronomical year numbering is used, thus 1 BC is 0, 2 BC is −1, and 4713 BC is −4712. JDN is the Julian Day Number. Use the previous day of the month if trying to find the JDN of an instant before midday UT.