Right way to convert string into NSDate?(iphone)

In 开发者_StackOverflowmy app I store the date as a string("MM/dd/YYYY" format). In the DB. later when I retrieve the string I have to compare two dates, how do I achieve this?

If I had used the"YYYY/MM/dd" format I could have directly compared it as strings. Now I have to convert back into NSDate object using "nsdateformatter" and "dateFromString". But I get a constant date value using this no matter what I do.

Any ideas??

NSDateFormatter* df = [[NSDateFormatter alloc] init];
[df setDateFormat:@"MM/dd/yyyy"];
NSDate* d = [df dateFromString:@"12/23/2046"];
NSLog(@"%@", d);

Result:

2046-12-23 00:00:00 +0800