How many days between two dates

Dates are stored as a count of days, so the gap between two of them is a subtraction. No function required: later date minus earlier date gives the number of days between.

The one thing to watch is the result's formatting. Subtract two date-formatted cells and Excel sometimes decides the answer should be a date too, and shows you January 1900 instead of 23. The value is right; the costume is wrong. Set the cell back to a plain number.