XMM-Newton Users Handbook

6.1.4 Time scale and Reference Time

The Time System used in XMM-Newton is Terrestrial Time or TT (TIMESYS keyword in FITS files headers):

TIMESYS = 'TT '

The Reference or zero time used (MJDREF keyword in FITS files headers) has been defined as:

MJDREF = 50814.0 (Modified Julian Date)

corresponding to:

1998.0 TT = 1998-01-01T00:00:00.00 TT = 1997-12-31T23:58:56.816 UTC

The conversion from TT to UTC can be found in The Astronomical Almanac. For this reference date it is:

TT = UTC + 63.184 sec

The use of this scale for XMM-Newton data gives rise to time values of the order of $10^8$ s.

Detailed information on time scales can be found, among other references in
http://aa.usno.navy.mil/publications/docs/Circular_179.php