[things that mystify me] Someone suitably old needs to explain this to me:

what is a daytime phone number?

Like I just assume if you have a phone number it works 24/7, sooo why daytime?

I think it means where to reach you during the day, like if you go somewhere for work

More or less what I was about to say.

so like the phone number still works 24/7 but you might not be there to answer it in the evenings

So with the advent of cellphones, this really IS an old thing. But some really do have a day phone and work phone.

Yeah, more for pre-mobile phones if you were working somewhere maybe during the day and you can be reached on home phone at night.

yeah, definitely less common and less relevant now lol

but I still see it on forms and applications and stuff from time to time

Yeah, it's a holdover from the era before cellphones becoming commonplace.