Excel DATEDIF Function

The DATEDIF (Date + Dif) function is a "compatibility" function that comes from Lotus 1-2-3. For reasons unknown, it is only documented in Excel 2000, but it works in all Excel versions since that time. As Chip Pearson says: DATEDIF is treated as the drunk cousin of the Formula family. Excel knows it lives a happy and useful life, but will not speak of it in polite conversation.

Time units

The DATEDIF function can calculate the time between a start_date and an end_date in years, months, or days. The time unit is specified with the unit argument, which is supplied as text.  The table below summarizes available unit values and the result for each. Time units can be given  in upper or lower case (i.e. "ym" is equivalent to "YM").

Basic usage

In the example shown above, column B contains the date January 1, 2016 and column C contains the date March 1, 2018. In column E:

E5=DATEDIF(B5,C5,"y") // returns 2
E6=DATEDIF(B6,C6,"m") // returns 26
E7=DATEDIF(B7,C7,"d")// returns 790

Difference in days

The DATEDIF function can calculate the difference between dates in days in three different ways: (1) total days , (2) days ignoring years, and (3) days ignoring months and years. The screenshot below shows all three methods, with a start date of June 15, 2015 and an end date of September 15, 2021:

The formulas used for these calculations are as follows:

=DATEDIF(B5,C5,"d") // total days
=DATEDIF(B6,C6,"yd") // days ignoring years
=DATEDIF(B7,C7,"md") // days ignoring months and years

Note that because Excel dates are just large serial numbers, the first formula does not need DATEDIF and could be written as simply the end date minus the start date:

=C5-B5 // end-start =  total days

Difference in months

The DATEDIF function can calculate the difference between dates in months in two different ways: (1) total complete months , (2) complete months ignoring years. The screenshot below shows both methods, with a start date of June 15, 2015 and an end date of September 15, 2021:

=DATEDIF(B5,C5,"m") // complete months
=DATEDIF(B6,C6,"ym") // complete months ignoring years

Difference in years

The DATEDIF function can calculate the difference between dates in complete years with just one method, shown below:

=DATEDIF(B5,C5,"y") // complete years
=DATEDIF(B6,C6,"y") // complete years
=YEARFRAC(B7,C7) // fractional  years with YEARFRAC

Notice in row 6 the difference is almost 6 years, but not quite. Because DATEDIF only calculates complete years, the result is still is still 5. In row 7 we use the YEARFRAC function to calculate a more accurate result.

Notes

1. Excel will not help you fill in the DATEDIF function like other functions.
2. DATEDIF with throw a #NUM error if start date is greater than the end date. If you are working with a more complex formula where start dates and end dates may be unknown, or out of bounds, you can trap the error with the IFERROR function, or use MIN and MAX to sort out dates.
3. Microsoft recommends not using the "MD" value for unit because it "may result in a negative number, a zero, or an inaccurate result".