When you open a Microsoft Excel file in Apple Numbers date columns may be converted to numbers with 5 digits. That is because in Excel dates are stored as the number of days since 1900-01-01. Converting takes setting the column type to duration in days and in a second column add the days to that special date.
In the steps below I assume the column to convert is named A and the new column is B.
- Select column A
- On the sidebar:
- Cel > Data Format > Duration > Custom Units > Day
- Add a new column B
- Insert formula:
- Select column B
- Menu Table:
- Autofill Cells > Autofill Down
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