The Time System used in XMM-Newton is Terrestrial Time or TT (TIMESYS keyword in FITS files headers):
The Reference or zero time used (MJDREF keyword in FITS files headers) has been defined as:
The conversion from TT to UTC can be found in The Astronomical Almanac. For this reference date it is:
The use of this scale for XMM-Newton data gives rise to time values of the
order of s.
Detailed information on time scales can be found, among other references in