Simply take a callback as you normally might, when getting the input from the caller on their desired callback time calculate the delta time ( you might consider factoring in EWT) and delay until then before queuing.
For web callback, if you have Premium use HTTP triggers and standard web invocation methods. If Enhanced then you'll have to be a lot more creative or use a solution such as ours which can facilitate the callback and invoke the applications on the UCCX.
Hopefully that's enough to get you going in the right direction.
Tanner Ezell
www.ctilogic.com