ISDC Julian Date

The Julian Date is a standard way of expressing Earth time. The definition of the Julian Date depends however on the time system, i.e., there are different possible Julian Dates. ISDC has chosen to use the Terrestrial Time. The Julian Date is defined as the number of days elapsed since December 31 of year -4713 at noon. Unfortunately, the present Julian Date is about 2.5 million days. Expressing times with millisecond accuracy would require 17 significant digits, which does not fit in a double precision real. The Modified Julian Date (MJD) is equivalent to the Julian Date -2 400 000.5 days. Current MJD is about 55000, and two digits can be spared. This would marginally fit in a double precision real.

To avoid any accuracy problem caused by the storage of the data in a double precision real, the ISDC Julian Date (IJD) was defined, which is the Julian Date, but with a reference close to the date of the INTEGRAL mission. The 1st January of Year 2000 at 0h 0m 0s Terrestrial Time has been chosen as the reference Julian Date.

Since Terrestrial Time differs from UTC by 32.183 sec + 32 leap seconds at the start of year 2000, the UTC origin of the IJD is actually 1999-12-31T23:58:55.817. IJD is connected to the MJD in the following way: IJD = MJD - 51544. The tool converttime distributed with OSA performs the time conversions from different systems.