Your cells are formatted as text, not as dates; hence the content is text, not dates (dates would be right-aligned).

First, change the cell format to your required date format for all the dates.
Next, input 1 into any other cell, then cut that to put it in the clipboard.
Finally, select all the dates and choose Paste Special>Values>Multiply>OK.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]