It's been since Katrina slammed into Mississippi and Louisiana on Aug. 29, 2005 at about 9am.
It's been since a tornado and windstorm hit north Jackson around noon on April 4, 2008.
The worst terrorist attack on American soil took place ago.

Unless you select the last day of a month, a simple subtraction finds the number of days. If the day of the end date is less than that of the beginning date, the beginning day will be added to the number of days in the beginning month before the subtraction is made.  Note the difference between Jan 31 and Feb 28 will be one month in non leap years but 28 days in leap years.  If both dates are on the last day of the month, the difference in days will be set to zero--the only difference will be between years and months.  No adjustment is done for longer or shorter months.