NSString -> NSDate 변환

 NSDateFormatter *dateformat = [[NSDateFormatter alloc] init];

// AM,PM으로 할 경우 hh

// 24시간으로 할 경우 HH

[dateformat setDateFormat:@"yyyyMMddhhmmss"];
           
NSDate *beginTime = [dateformat dateFromString:@"20120101000000"];
NSDate *endTime = [dateformat dateFromString:@"20121231235959"];
NSDate *currentTime = [NSDate date];