This is because you’ve accidentally input the values as text somewhere along the line. First make sure your format is set to date in the range in question.
Make use of the DATEVALUE formula in blank column =DATEVALUE(your_range). This will output your range as a number. Note that your_range needs to be in your computers date format (I have mine set to YYYY.MM.DD, so that what I have to use, else it won’t work. You might be American, so your dates would need to be MM/DD/YYYY) for DATEVALUE to recognise the text as a date string.
Then copy the output, paste as values only (alt, h, v, v), then copy that and paste it back into you range.
Make sure the format is set to date and you’re laughing.