How many days until december

So, I’m working on an app that will be a calendar with different events for different people.
The problem is that I have no idea how many days Until december 28 to calculate the days from now until the date of the event.
For example, if I set the start date to November 9th and the end date to December 1st, is there a formula or some sort of code to make sure that the end day is not before the beginning day?
If I did not know the number of days betwe