DAYS calculates the difference between two date values. The result is an integer and returns the number of days between the two days.

DAYS(`date_2`,`date_1`)

`date_1`is the start date.`date_2`is the end date.

If `date_2` is an earlier date than `date_1`,
the result is a negative number.

`=DAYS("1/1/2015",NOW())` returns
the number of days from today until 1 January 2015.

`=DAYS("10/10/1990","10/10/1980")` returns `3652`.