You could have a field where the employee's entry is recorded, then you could have a second field that is rounded.
Maybe something like
Round(ToNumber(EmpTime),.25)
You'd have to turn it back into a time
Idk if you can round times w/o turning it to a number?
There's prob a cleaner solution but at least that's something.
And no i didn't test it, it's 1 AM and I need to go home.
Let me know how it goes though :)