|A few things to try:|
I - Make sure that the sheet is not set to display formulas.
II - If that doesn't help, try this:
1 - Copy a blank cell
2 - Select the date cells
3 - Edit > Paste Special, Add, Values.
4 - Format as date
III - If that doesn't Help, try this:
1 - Select the date cells.
2 - Data...Text To Columns...Delimited...Next...Next...Finish
3 - Format as date
IV - If that doesn't work, try the second suggestion again. (Suggestion 3 may force the values to be text, adding a blank cell will turn them back into numbers.
Remember to check the format to make sure it is set to Date after trying anything. The format may have changed to something else without you knowing it.
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