Have it a try. "mm" stands for minute while "MM" stands for month.
NSDateFormatter * dateFormatter = [[NSDateFormatter alloc] init] ;
[dateFormatter setDateFormat:@"yyyy-MM-dd"] ;
NSDate *date = [dateFormatter dateFromString:@"2014-01-23"] ;
NSLog(@"date=%@",date) ;
NSTimeInterval interval = [date timeIntervalSince1970] ;
NSLog(@"interval=%f",interval) ;
NSDate *methodStart = [NSDate dateWithTimeIntervalSince1970:interval] ;
[dateFormatter setDateFormat:@"yyyy/MM/dd "] ;
NSLog(@"result: %@", [dateFormatter stringFromDate:methodStart]) ;