Dates in Excel are serial numbers that start on 1/1/1900, which is 1.
In the example, the formula in cell D6 simply subtracts the numeric value of 1/1/1999 (36161) from the numeric value of 1/1/2000 (36526) to get a result of 365. The steps look like this:
=C6-B6 =36161-36526 =365
Working with today
If you need to calculate the number of days between an earlier date and today, you can use the TODAY function:
To calculate the number of days between a later date and today, use:
Note that TODAY will recalculate on an on-going basis. If you open the workbook at a later date, the value used for TODAY will update and you will get a new result.
The DAYS function
The DAYS function, introduced in Excel 2013, also calculates days between two dates.