从头信息中拿到的Date通常是这样的：
Date = "Sun, 12 Apr 2015 03:16:27 GMT";
需求是将时间字符串转为时间戳
- (void)toDate:(NSHTTPURLResponse *)response
{
NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
[formatter setTimeStyle:NSDateFormatterFullStyle];
[formatter setDateFormat:@"EEE',' dd' 'MMM' 'yyyy HH':'mm':'ss zzz"];
[formatter setLocale:[NSLocale localeWithLocaleIdentifier:@"en_US"]];
[formatter setTimeZone:[NSTimeZone timeZoneWithName:@"Asia/Shanghai"]];
[formatter setCalendar:[NSCalendar calendarWithIdentifier:NSGregorianCalendar]];
NSString *string = response.allHeaderFields[@"Date"];
NSDate *date = [formatter dateFromString:string];
NSInteger time = [date timeIntervalSince1970];
NSLog(@"%ld",(long)time);
}
