Dealing with typing archaic letters to get a specific date/time pattern is just wrong. And a link to a PHP manual is not the answer... unless you're a programmer.
Some options - there should be a list of common examples on that page (e.g. how do I add am/pm) - Replace the PHP.net tutorial link with one on bdcms.org that explains it better - get away from having the user deal with PHP stuff. Provide some kind of drag and drop or GUI to help. Not sure what at the moment.
This always takes me time to figure this out.