Convert from date string in unknown format

I have data that contains a date string.

Normally it would be in a 'Jan. 3, 1966' type format, but because of international differences, it may not always be exactly that.

I need to read the data in and convert it into a standard date string ('YYYY-MM-DD').

Basically this is what I have so far:

var dataString = 'Jan. 3, 1966'  
var dateFormatter = NSDateFormatter()  
dateFormatter.dateFormat = # I DON'T KNOW THE EXACT INPUT FORMAT !
let dateValue = dateFormatter.dateFromString(dataString)