Nick said “The FITS “standard” is irritatingly vague about things like this ..”
According to this document, in 1997 the IAU recognised this confusion and standardised DATE-OBS as the start time
https://www.cv.nrao.edu/fits/documents/standards/year2000.txt
“4.2) Henceforth, DATE-OBS shall be assumed to refer to the start of an
observation. Other interpretations must be clearly explained in the comment field.”
Cheers
Robin