Its also worth remembering – for astrometry certainly – that though a piece of software may say that a time given is the start, end or middle of an exposure it may not be as accurate as you would expect.
I recall some of the guys from Basingstoke society devising ingenious ways to determine how accurate the recorded time of a DSLR exposure was and finding not just the shutter/camera OS delay time, but also delays of more than a second in some commercial camera control software – which would play merry hell with NEO measurements.
The told the developers of one of the software packages, but they didn’t seem to care.