The time stored in a Miriad visibility dataset is nominally UT1 (although
the difference between UT1 and UTC is not significant for the purposes of
Miriad). As Miriad relies on correct time in a number of calculations
(e.g. computing the parallactic angle needed in polarization calibration and
conversion, computing velocity information in fits
and recomputing u-v coordinates done by uvedit),
does its best to convert the time into UT1. fits
will give you messages about any time adjustments it makes.
When a FITS file contains multiple antenna tables,
the times stored in the FITS file have usually been offset from the true
time by the AIPS task DBCON. Task
assumes that such an offset has been added, and corrects for it.