To change Date format in a disorderly column

[p45cal]

I suspect this is happening because you are importing or copy/pasting the data from somewhere else into Excel.
Excel is being 'helpful' by trying to convert dates into real excel dates as the import/paste is carried out. It's making assumptions about the months and days, that is it's assuming that the dates are mm/dd/yy until an impossible month greater than 12 occurs.
One thing you can do to prevent Excel from doing this is to format the cells you're copying to as TEXT before pasting the dates. Then you'll be able to use a trick with Text to Columns to convert the dates to real dates (everything remains in the same single column)
Another way depends on how you're importing the text - you will probably be able to tell Excel the date format as you're importing.

So the questions arise:
1. Is this indeed caused by getting the dates etc. from somewhere else?
2. If so how are you currently importing the data and from where?