Alright, so lets say that my quest tells me to come back in a day's time. I set a short variable "day" to GameDay and then test if GameDay >= day + 1. This works perfectly, as long as it isn't the end of the month.
If, for example, it is the final day of the month, the day variable will have a value of 31, for example, while the GameDay will never be above that (as 31 is the top value). Any suggestions? I suppose I might also test for the month, but I suppose that once if it gets the value of 12 I have the same problem.

Use http://cs.elderscrolls.com/constwiki/index.php/List_of_global_variables.