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Try changing the format of the Date Duration cell to Number, as the number result 30 in Date format are 30 days after the first date that Excel accepts (which is 01/01/1900), so it is day 30/01/1900.
And the dates E2 and F2, in number format are: 42826 and 42856, respectively.
For example, with formatting as number cells look like this:
And as dates, so:



It gave good, thank you very much. Put as reply !
– Andre