Local "time" variable defined as TIME vs defined as NUMBER and using Time Difference (math)

So I want to do the "time difference" math on the following variables-

closetimeyesterday, currently defined as TIME (let's say it's 20:30)
closetimetoday, currently defined as TIME (let's say it's 20:00)

and put the result in-
closetimedelta, defined as....well, I thought should also be TIME, but read on

with which I then want to do a comparison to see if closetimetoday is -30 minutes or more EARLIER than closetimeyesterday, or +30 minutes or more LATER

If I define closetimedelta as a TIME variable I do not get the option to do a TIME DIFFERENCE, if I define it as a NUMBER I DO get that option.

and I'm not sure at this point if I'm wanting that to be 00:30 or +/-30

Throwing this out there for a few reasons:

1. for an explanation as to why the logic would not follow that ALL subject variables are TIME and that being allowed to do the Time Difference in the menu pick should follow

2. in case others have dabbled in comparing times & differences might know that could short circuit my trial & error efforts, specifically with the expectations of comparing "00:30" minutes as time vs "30" minutes as a number. (Typing this out I'm thinking, "well of course you just want THE NUMBER at this point to be able to do a comparison.")

3. for clarification on the wisdom of using TIME variables and offered TIME DIFFERENCE function vs converting everything to EPOCH TIME and then doing straight VARIABLE MATH

Thanks in advance.