I get the #NAME? error when I use the Days() function because the formula was built in Excel 2016 and it was probably not present in Excel 2007 yet. The error is shown:
Originally, the formula suddenly had a xlfn appear before the function, I do not know why so I removed it.
These all produce the same result.
=AE2-AD2
=DATEDIF(AD2, AE2, "d")
=DAYS(AE2, AD2)
If you need to add 1 to make the count inclusive then just tack on +1
or use,
=NETWORKDAYS.INTL(AD2, AE2, "0000000")