I recall a couple of old SciFi stories about time, one where folks jump from Monday to Wednesday without realizing Tuesday went missing, another where the MC finds himself in the space between Monday and Tuesday. Stuff ensues.
Let's give the OP idea that F&SF treatment. The hooker books sessions on impossible days -- and those days happen anyway. (How? An interdimensional datebook.) She builds a lucrative trade, doing hubbies when nobody will miss them. She likely charges bonus rates for 29-30 February.
Then the wondrous time structure collapses (like an unstable tessarect) and she's forced to double-up all her 31st johns on the 30th of April, June, September, and November. Yeah, much spit-roasting.