Not sure what you mean by "flag up"

but if you have a cell with the invoice date and you want another cell to
tell you if it's overdue you can use the following

assume cell A1 has invoice date
"certain time"=2 weeks(14 days) in my example

=IF(TODAY()-A114, "OVERDUE","")

If this is not what you were looking for, please clarify your question.

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"bgc1416" wrote:

i want an invoice that will flag up when customer not paid within a certain
time, any ideas