It's 'til, isn't it? I just now thought that through. 'til is an abbreviation of the word until, which means it has an apostrophe and one l. Till is what you do to dirt. So it's not "a quarter till", it's "a quarter 'til". I wonder how long I've been typing that and making peoples ignorant alerts go off.
Or, not. Merriam Webster has the word "till" as an abbreviation for "until". Oh, who the hell knows.