Date filter problem

hi, I have a bit of a strange problem, sometimes when I have a column of dates on an excel spreadsheet that are in the standard english format dd/mm/yyyy when i filter that column and put them in accending order, instead of filtering them in cronological order it filters them in number order for example 01/01/12 followed by 01/02/12?? this dosent always happen, and I have checked in the format cells option that the cells are formated as "date"
Does anybody know what I am doing wrong?
Regards
Martin

Maybe some of your dates are formatted as mm/dd/yyyy.

Pete