different time system.

http://cxc.harvard.edu/contrib/arots/time/time_tutorial.html

CHandra, XMM-Newton, and RXTE all have used the TT (Terrestrial Time) system.
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Chandra FITS Tables

The time stamps presented by Chandra FITS tables as generated by XFF will be in TT.

It has been agreed that the Spacecraft Clock Seconds (SCCS), or Mission Elapsed Time (MET), will represent true elapsed seconds since January 1, 1998, at 0h0m0s TT, which corresponds to MJD = 50814.0 (TT). At this time, the leapsecond value (TT - UTC) was 63.184 s. Hence:

MJD (Jan 1, 1998, 0h TT) = 50814.0 (TT) = 50813.999268703704 (UTC)

Raw time stamps in the FITS tables created by XFF will be on this (SCCS or MET) time system. However, FITS times are to be taken as the sum of a Time value and the values of the MJDREF and TIMEZERO keywords.

The TIMEZERO keyword will provide a nominal clock correction that ensures an absolute accuracy of TBD (nominally 1) ms for the time stamps. The exact value of the tolerance will be based on experience in orbit; the goal is to provide reasonably accurate absolute time with a minimum effort.

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http://heasarc.nasa.gov/docs/xte/abc/time_tutorial.html#systems

RXTE FITS Tables

The time stamps presented by RXTE FITS tables as generated by XFF are in TT.

It has been agreed that the Spacecraft Clock Seconds (SCCS), or Mission Elapsed Time (MET), will represent true elapsed seconds since January 1, 1994, at 0h0m0s UTC, which corresponds to MJD = 49353.0 (UTC).

Raw time stamps in the FITS tables created by XFF will be on this (SCCS or MET) time system. However, FITS times are to be taken as the sum of a Time value and the values of the MJDREF and TIMEZERO keywords. The MJDREF keyword will incorporate the transformation from UTC, via TAI, to TT (equaling an offset of 60.184 s). Thus, for January 1, 1994, at 0h UTC:

MJD (Jan 1, 1994, 0h UTC) = 49353.0 (UTC) = 49353.000696574074 (TT)

The TIMEZERO keyword will provide a nominal clock correction that ensures an absolute accuracy of 60 µs for the time stamps. The goal is to provide reasonably accurate absolute time with a minimum effort. Generically, the value of TIMEZERO may be described as "the fractional part of UTCF".

[UTCF is an institutional dinosaur that can be found in the telemetry. Its purpose is to provide a conversion to UTC: when the sum of SCCS and UTCF is divided by 86400, the integer part of the result provides the number of UTC days since January 1, 1993, at 0h UTC, while the fractional part indicates the current UTC. Thus, UTCF is the sum of the number of seconds in 1993 (31,536,001) and the clock correction, minus the number of leap seconds since January 1, 1993.]