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Hi Kristine,
I agree with Joeu's suggestion, and you could try his formula or share more information as he mentioned. Since we don't know the content and cell format of C3, as well as any error message like #VALUE ?
To add a point, it's not suggested to enter content in this bracket TODAY(), if you want to modify the date, you might select a cell then input the date into it the same way you choose C3. For reference, you could view this page: DATEDIF function
Please feel free to post back with a sample or a screenshot if the problem remains, and I would initiate a PM invitation for you, you could send me the sample or share the file via this link.
Best Regards,
Oliver