Excel date format issues

Yes, that sometimes happens here too.

Assuming the Excel date serial number in A2, you can convert to a Numbers date-time with this:

=DATE(1900,1,1)+A2−2

Replace the , in the formula with ; if your region uses , as a decimal separator.

Once the numbers are converted to dates, select the dates, command-c to copy, and Edit > Paste Formula results. Then delete the column with the date serial numbers.

And there is also another way, using a short script. The advantage is this one is you don't have to add an extra column, enter a formula, and do housekeeping afterward.

Step 1: Copy-paste script below into Script Editor (in Applications > Utilities)

Step 2: Select the cells with the serial number dates

Step 3: Click the <run> button in Script Editor

If "nothing happens" make sure Script Editor is listed and checked at System Preferences > Security & Privacy > Privacy > Accessibility.

Since this changes the values in your table I suggest testing on a copy of the table first to make sure it is doing what you want it to do.

The script:

set startdate to date "Monday, January 1, 1900 at 00:00:00"

tell application "Numbers" to tell front document
	tell active sheet
		tell (first table whose class of selection range is range)
			repeat with c in (get selection range)'s cells
				set value of c to startdate + ((value of c) - 2) * days
			end repeat
		end tell
	end tell
end tell

You can give feedback to Apple about this problem via Numbers > Provide Numbers Feedback in your menu.

SG