Green’s Dictionary of Slang

jelly-date n.

also cake-date, coke-date
[the relevant SE food; jell v. + date n.1 ]
(US campus)

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

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.
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.
[US](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.
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.