Excel Dates

Thank you for your fast response.
i did this and it says
"Symptom
Excel displays the #VALUE! error in one or more cells in a worksheet.

Causes
One or more cells that are included in a formula contain text, and your formula performs math on those cells by using the standard arithmetic operators (+, -, *, and /).
For example, the formula =A1+B1, where A1 contains the string "Hello" and B1 contains the number 3, returns the #VALUE! error.

A formula that uses a math function, such as SUM, PRODUCT, or QUOTIENT, contains an argument that is a text string instead of a number.
For example, the formula PRODUCT(3,"Hello") returns the #VALUE! error because the PRODUCT function requires numbers as arguments.

Your workbook uses a data connection, and that connection is unavailable.
Example
When you copy the example data to a blank worksheet, the formulas in cells A4 and A5 return a #VALUE! error; however, cells A6 and A7 return the correct value of 30."

which goes against what i did in my training course and what i was told.
Thanks
fi