Updating a cell, but stopping the update after a certain Timestamped date

[Darknyte]

Hello, I am using Timestamp (NOW()) to fill out an IF function:
=IF(AND(S21>=O37,S21<S37),S17,0)

Where S21 is my Timestamped date, O37 is my minimum date and S37 is my max date, S17 is a percentage that updates as data is updated. The data updating is what updates the timestamp. Instead of 0 in the FALSE statement, however, I would like for it to take the last known reading of S17 and keep the cell populated.

In easier terms, when the administrator updates the cell to show completion of work, it updates the date when she updates the cell. To show progression of work completed, I have different tables showing progression as of a certain date (row 37). If I could get this to work, she would only literally have to enter in the data in one or two columns and it would automatically show progression as time went along.

anyone?

 

[Darknyte]

=IF(T17>0,NOW(),"") Where T17 is a cell that is the Total of a row of data entered by our administrator. I am finding out, though, that wherever I update the worksheet, it updates the timestamp, not just when the cell T17 is greater than zero!.. HELP!