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 difference between TT and UTC depends, among other things, on the leap second. As of July 1, 2015 we have:
The use of this scale for XMM-Newton data gives rise to time values of the
order of 1.0E+08 s.
Detailed information on time scales can be found, among other references in
A time conversion tool can be found under