Date format

Hello everyone!

I am working on a big sheet for a research project involving dates. I did not pay attention, and now some dates are entered in the english format (ex: 2013-10-15 converted to 10-oct-2013 with the "date option") and now I have entered dated in the francophone format (ex: 01.02.2014 for the 1st of February 2014) and now when I try to convert these dates to the same format (day (in numbers) - month (first 3 letters) - year (last 2 numbers)) it doesn't seem to work...

I want the same format because I'm going to need to calculate the number of years between two dates.

Thank you!
Julien Prévost

Thank you,
I actually used the "text to column" and its "date" feature. Now Excel sees my numbers as dates.
Cheers,
Julien