@abdallahh

Most likely due to the fact that your dates are in the wrong format. Which locale does your setup expect?

dd/mm/yyyy or mm/dd/yyyy

I suspect the latter, so when you have a date 13/1/2014 (13 January) your system errors out as is doesn't recognise a 13th month. For you, 12/1/2014 is probably December 1, not 12 January. 

When you change the data type you can choose "Locale" and then a European one or the one for the US. So, if you are dealing with European dates on a US system, you need to choose a European local. Perhaps you can upload a file with your date. Then it's easier to see what's needed.

Change Type Using Locale