The ONTIMEn keywords give the sum of the good time intervals (GTI) for ACIS CCD_ID = n, where n = 0, 1, 2, ..., 9. The value of the ONTIME keyword equals the ONTIMEn value for the CCD at the aim point of the telescope.

  ONTIMEn = sum of GTIs

The GTIs should be integer multiples of TIMEDEL + FLSHTIME (i.e. integer multiples of the total time per frame). However, constraints on the quality of the aspect solution can lead to GTIs that include fractional portions of TIMEDEL + FLSHTIME. Since the GTIs differ from CCD to CCD, there is one ONTIMEn keyword for each CCD used for an observation.

If an observation uses interleaved mode then there are separate values of ONTIMEn and ONTIME for the short and long frames.

The observation times entry and the chandra times help file explain how the various exposure keywords are related.