how create delta T from seperate date string and time string

I've got a 1000x2 cell structure 1st column containing date 2nd containing time collected from an IMU

the delta_T is different for every reading, ie it does not read every .015 seconds. I would like to create a variable to track this different delta_T:

I've thought to combine the date and time via datenum, but can't figure out how to do that, then subtract the difference and convert it back to real time.

any help or different routes are appreciated

[Sarah Wait Zaranek]

What format is your time and date in? I can give you an example, but it would better if I knew the format.