The Fermi Science Tools use mission elapsed time (MET), the number of seconds since the reference time of January 1, 2001, at 0h:0m:0s in the Coordinated Universal Time (UTC) system, corresponding to a Modified Julian Date (MJD) of 51910 in the UTC system.
The Systeme International (SI) second is defined as the duration of 9,192,631,770 cycles of radiation corresponding to the transition between two hyperfine levels of the ground state of cesium 133.
Universal Time 1 (UT1) is the time system based on the rotation of the earth. Because of changes in the earth's rotation rate, in UT1 a day is not exactly 86400 s.
Coordinated Universal Time (UTC) provides a uniform-rate time system referenced to atomic clocks where a day is 86400 s. To keep UT1 and UTC within 0.9 s, a leap second is added to UTC as needed, typically every few years (see the USNO leap second history). UTC is the same as Greenwich Mean Time (GMT) or Zulu time (for the military).
Leap seconds can cause errors in measurements that straddle the addition of a leap second. The Terrestrial Time (TT) is a uniform rate time system referenced to the geoid without leap seconds. Effectively, TT time is greater than UTC time by a number that increases by 1 second every time a leap second is added to UTC. For Fermi's reference time of January 1, 2001, the difference was 64.184 s.
Another method of avoiding leap seconds over a time span of a number of years, such as the duration of the Fermi mission, is to use the number of seconds relative to a reference time. This is the method that astrophysical missions often use, where the number of seconds is called 'Mission Elapsed Time' (MET). Because MET and TT are both continuous uniform-rate time systems, MET and TT will always be offset from each other by a constant. Note that the time system should be included in specifying a reference time (e.g., whether the reference time is midnight on a particular date in the TT or UTC system).
The Global Positioning System (GPS) uses its own continuous uniform-rate time system that is related by a constant offset (13.184 s) to TT. The Fermi spacecraft uses a clock that is synchronized with signals from GPS satellites, and tags both housekeeping and science data with MET relative to January 1, 2001, 0h:0m:0s UTC.
The Julian Date (JD) is the number of days since Greenwich mean noon on January 1, 4713 B.C.E. Since JD is a large numbermidnight at the beginning of January 1, 2008, corresponds to JD=2454466.5and our calendar uses midnight as the beginning and end of a calendar day, the Modified Julian Date (MJD) has been defined as MJD=JD-2400000.5. Midnight (i.e., 0h:0m:0s) differs between the UTC and TT systems, and therefore one should specify whether a JD or MJD date is in the UTC or TT system.
The interface to the Fermi Science Tools uses MET, the number of seconds since midnight at the beginning of January 1, 2001, in the UTC system. Time is represented in the science data products as a double precision MET in seconds from the reference time given by the MJDREFI (the integer part) and MJDREFF (the fractional part) keywords in the FITS header. Although we use MET, which does not include leap seconds, we nonetheless want the science data to be in the TT system. Since the reference time is midnight in the UTC system, MJDREFF is non zero in the TT system. The same MJDREFI and MJDREFI is used by all science data products for both the GBM and the LAT. For our choice of reference time:
In addition, the spacecraft clock drift for data obtained during periods when the GPS time signal is not available from the spacecraft can also be specified.
Because UTC corresponds to the time kept by everyday clocks, after correcting for the time zone, the ground system will use UTC.
In analyzing Fermi observations you may need to convert between MET and other time expressions such as Year-Month-Day, Year-Day of year, or MJD. The HEASARC provides a useful conversion tool for different missions including Fermi. Note that Swift also uses midnight, January 1, 2001, UTC as a reference time, and therefore the Swift and Fermi MET will be the same.