I have all my dad's slides, dated from 1955 to the late '60s. Sometimes, he would take a roll of 36x film over the course of an entire year. There's no way for the lab to know that frames 1-4 were taken in January, 1963, frame 5 was in February, frames 6-9 were taken in May,................
If he took the film in for processing in March 1964, they have Mar 64 stamped on 'em.
Only his hand-written notes on the frames has the correct date. Usually, just the year ("Vacation, 1961").