Chances are that the ones that are frozen are really text strings, not dates
(you could check by applying the comma style to those cells; if they're
dates, you'll get some really big percents. If not, you still won't see a
change). If that's it, adding 0 to these cells will help Excel convert them
to numbers (and dates are really just specially formatted numbers). Type a 0
in some random cell and copy it. Select the column with the dates and Edit >
Paste Special, select the 'Add' radio button and click OK. Everything will
probably show up as a number in the 30000 range. Now apply your preferred
format.