From Microsoft Excel 365 Training Johannesburg
You have to enter your date in "Date Format" in order to use it in a formula.
Excel recognizes the following formats as dates: 28 Oct 2018, 28 October 2018, 28 Oct 18, 2018/10/28
If you have an apostrophe ' before your date, Excel would recognize it as a label, and not as a value that you can use in formulas. We are going to calculate how many days you are old in cell C4 (DOB = Date Of Birth).
Row Colunm A Column B Colomn C
1 Start date End Date No of days
2 30-Oct-18 31-Oct-18 =B2-A2
3 30-Oct-18 02-Nov-17 =B3-A3
4 own DOB Date today =B3-A3
From Advanced Excel for the Power User