Update a formula based on date worksheet was saved
 

I'm trying to update a weekly report that contains historic data.
the challenge is to count only the new information based on the date the
worksheet or workbook was saved as.
For example if the worksheet was saved as worksheet 9.30, I want to count
and update only the data entered after the saved date of 9.30. Because the
report is weekly, I need to update on a dynamic basis.
Any help will be greatly appreciated.

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KevinM