Julian Dates
Julian Dates and Modified Julian Dates are used as systems of continuous time allowing the analyzes of time-varying
astronomical data.
1. JULIAN DAY NUMBER (JDN)
The Julian day number is the number assigned to a day in a continuous
count of days beginning with the Julian day number 0 assigned to the day
starting at Greenwich mean noon on 1 January 4713 BC, Julian proleptic calendar
(-4712).
2. JULIAN DATE (JD)
The Julian Date (JD) of any instant is the Julian day number for the
preceding noon plus the fraction of the day since that instant. A Julian
Date begins at 12h 0m 0s and is composed of 86400 seconds. To determine time
intervals in a uniform time system it is necessary to express the JD in a
uniform time scale, which is Terrestrial Time (TT) where the length of day
is 86,400 SI seconds. Note that UT (and UTC) is not continous (leap seconds!)
so the beginning of the Julian day differ from UTC noon by about half a minute.
3. MODIFIED JULIAN DATE (MJD)
MJD = JD - 2 400 000.5)
4. TRUNCATED JULIAN DATE (TJD)
TJD = JD - 2,440,000.5 = MJD - 40,000
Online
conversion tool at HEASARC/GSFC