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 polarisation calibration and conversion, computing velocity information in fits and uvredo
and recomputing u-v coordinates done by uvedit ), task fits 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 fits assumes that such an offset has been added, and corrects for it.