change from vba code (const as date) to (cell.value in worksheet)

Hello,
I have one new issue.
When I'm trying to change to cell value I've got Global Error.

Code:

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Const FirstDay As Date = "2017-12-31"

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to something like this:

Code:

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dim FirstDay as string
FirstDay = Sheets("DATA").Range("J3").NumberFormat = "yyyy-mm-dd"

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Thanks in advanced


ps
I change FirstDay as date and it's working.