I'm looking for an Excel user defined function to get the number of days between any two dates?

The dates may be far in the past or future such as: 4/7/1966 and 6/21/22451 or 7/18/1066 and 2/3/2014 or 6/15/-15384 and 2/6/0356 or 3/13/-625784 and 7/7/-625782 I'm been trying to build such a...