How to calculate the number of days between two given dates? (Leap year obstacle)

Question

• Any year evenly divisible by 400 is a leap year (e.g., 2000 was a leap year).
• Any other year evenly divisible by 100 is not a leap year (e.g., 1700, 1800 and

Answer 1

Convert them both to UNIX epoch time and subtract the difference.

UNIX Epoch time is the total number of seconds for a date since 1 January 1970 00:00:00.0

Once you've got the number of seconds, you divide that difference by the number of seconds in a day (which is 24 hours * 60 minutes * 60 seconds = 86400 seconds).