Excel shows ##### instead of a date? Check the column first

Move to the right border of the column letter. Double-click that border to fit the contents, or drag it to make the column wider. If the expected date appears, the cell had too little space to show it. If you need to keep the column narrow, select the date cells and choose Short Date from the Number Format menu on the Home tab.

If width made no difference, reference the affected cell from a spare cell and set the spare cell’s number format to General. This leaves the original format in place while you check the value. In a workbook that uses the 1900 date system, a negative value formatted as a date or time can display as hash marks. Check the subtraction order and the dates or times that feed it.

For example, an actual finish date earlier than the target can produce a negative result when you subtract the target from the actual date. If that sign is intentional, display the difference as a number of days. If a time difference needs to be shown in minutes, convert the day fraction to minutes and use a number format. Confirm the result against the original entries before changing the formula.