It depends on if you are speaking it or writing it. I would write March 18th, 2015. However (and I'm not 100% sure about this), I think in America people write March 18, 2015 and it's fine. If you're speaking it, I would say 18th of March, 2015. Hope that helps :)

US usage is "March 18, 2015" it is usually said as "March eighteenth". "The eighteenth of March" is also used, but is more formal in tone. We would usually only write "March 18th" if the year were not expressed, but we will also just write "March 18." You will very rarely see the form "18 March 2015" written by a US native. We abbreviate all days in the sequence m/d/y, or here "3/18/15" which is not the sequence in most other countries.