I do not understand, what is the "old vfp application" here and what is "some formula".
Where is this "formula", in your code, is it Excel cell formula, or what?
What does this formula do?

Give details and code for what is working in "old vfp", and what does not work in NET; then we will start from there.

>In my old vfp application, the excel run very well,my client use our excel and use some formula to procedure a new spreedsheet.
>
>however, In vb.net due to the "period" format, his formula can't run very well.
>I trr your method, However, when user remove the space , the column change back to Date format.
>If I add "'" to the field, user also complaint that his formula can't run.