The BeSS fits standard used in the spectroscopy database specifies optionally DATE-END for the end time but I think it is rarely used and DATE-OBS (start time) and EXPTIME is normally used
“Date-Time of start of exposure ● Keyword = DATE-OBS (mandatory except if DATE-END and EXPTIME are present) ● Format: char (70 char max) ● Mandatory format: yyyy-mm-ddThh :mm:ss[.ss…] ● Example: 2006-02-25T13 :34 :43.5543
Date-Time of End of Exposure ● Keyword = DATE-END (mandatory except if DATE-OBS and EXPTIME are present) ● Format: char (70 char max) ● Mandatory format: yyyy-mm-ddThh :mm:ss[.ss…] ● Example: 2006-02-25T13 :34 :43.5543
Exposure Time ● Keyword = EXPTIME (mandatory except if DATE-OBS and DATE-END are present) ● Format: float ● Units: seconds ● Valid Range: >= 0