Unable to sort dates by oldest to newest

Hi

So here is the situation. I have an excel 2007 spreadsheet that has a column with dates that I need to sort from oldest to newest or vice versa. The dates were having a mixed format of mm/dd/yyyy and mm-dd-yyyy

Here's what I did so far:

1. Selected the column and formatted the cells to a standard mm/dd/yyyy
2. Did Text-to-Column > Finish
3. Even after step #2, I was still unable to sort them from oldest to newest.
4. So I went to Text-to-Column > Next > Selected Date > Finish
5. The column still wouldn't sort date wise. So I repeated the above process #4 and this time, I changed the date format to DMY and back again to MDY
6. This time I was getting the sort from Oldest to Newest option but all the dates didn't switch back to the same format. Some remained in mm/dd/yyyy while some changed to dd/mm/yyyy

The weird thing is that, if after I perform step #2 and copy the column to a new sheet, I would immediately get the sort datewise option right away, but it isn't happening on the existing sheet.

Please find attached a sample list of dates attached. No action has been performed on this column yet

Kindly advise where I am going wrong

Hope I was able to explain my situation properly