Green’s Dictionary of Slang

a date to take a girlfriend to the soda fountain or similar, where one sits and chats.

1940		J. Dillon Modern Acting 135: if I say I’d just as soon go on a jelly date, perhaps he’ll think I’m cheap.
1975		T. Williams Memoirs 27: Whenever I would go out on a jelly-date, as afternoon dates were called, and find myself unable to pay the cafe bill, I would write out a check.
1982		(con. 1949) B.A. Mason ‘Detroit Skyline 1949’ in Shiloh 41: A jelly date was a Coke date. She had jelly dates with Bob and Jim and Sam all on the same day.
1982		B.A. Mason Shiloh and Other Stories 41: A jelly date was a Coke date. She had jelly dates with Bob and Jim and Sam all on the same day.