in cocoa:
NSDateFormatter can't format with  "yyyy-MM-dd a hh:mm", 
it return 2013-12-05 10:48, lose 'AM' part.
but it work with "yyyy-MM-dd" a " or "yyyy-MM-dd hh:mm"
in ios:
"yyyy-MM-dd a hh:mm" can work, it return 2013-12-05 AM 10:48
in cocoa:
NSDateFormatter can't format with  "yyyy-MM-dd a hh:mm", 
it return 2013-12-05 10:48, lose 'AM' part.
but it work with "yyyy-MM-dd" a " or "yyyy-MM-dd hh:mm"
in ios:
"yyyy-MM-dd a hh:mm" can work, it return 2013-12-05 AM 10:48