NSDateFormatter and yyyy-MM-dd

I'm trying to take a NSString date in the format "2/22/11" and convert it to this format: 2011-02-22

This is my code:

NSDate *dateTemp = [[NSDate alloc] init];
NSDateFormatter *dateFormat = [[NSDateFormatter alloc] init];
[dateFormat setDateFormat:@"yyyy-MM-dd"];
dateTemp = [dateFormat dateFromString:newInvoice.date];
newInvoice.date = [dateFormat stringFromDate:dateTemp];

newInvoice.date starts as an NSString equal to "2/22/11". dateTemp ends up NIL. newInvoice.date ends up NIL as well.

I can't for the life of me figure out why.