NSDateFormatter returns unexpected results

问题:

Why does the following iOS 4.2 code return two different times?

  NSTimeZone *gmt = [NSTimeZone timeZoneWithAbbreviation:@"GMT"];
  NSDateFormatter *dateFormatter = [[[NSDateFormatter alloc] init] autorelease];
  [dateFormatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
  [dateFormatter setTimeZone:gmt];
  NSString* dateString = [dateFormatter stringFromDate:[NSDate date]];
  NSLog(@"Date/Time is %@", dateString);

  NSDateFormatter *inputFormatter = [[[NSDateFormatter alloc] init] autorelease];
  [inputFormatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"];    
  NSDate* date = [inputFormatter dateFromString:dateString];        
  NSLog(@"Date/Time is %@", date);

Returns:

2011-01-04 16:15:12.966 WA[687:207] Date/Time is 2011-01-04 21:15:12
2011-01-04 16:15:12.967 WA[687:207] Date/Time is 2011-01-05 02:15:12 +0000

The first value is expected, but I would expect the second to be the same.

Bruce

回答1:

Neither of your date formats appear to include the time zone, so you're likely getting the difference between your location and GMT.

回答2:

Since dateString is already GMT, the trick is to set the time zone to GMT on inputFormatter as well. This code works as expected:

  NSTimeZone *gmt = [NSTimeZone timeZoneWithAbbreviation:@"GMT"];
  NSDateFormatter *dateFormatter = [[[NSDateFormatter alloc] init] autorelease];
  [dateFormatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
  [dateFormatter setTimeZone:gmt];
  NSString* dateString = [dateFormatter stringFromDate:[NSDate date]];
  NSLog(@"Date/Time is %@", dateString);

  NSDateFormatter *inputFormatter = [[[NSDateFormatter alloc] init] autorelease];
  [inputFormatter setTimeZone:gmt];
  [inputFormatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"];    
  NSDate* date = [inputFormatter dateFromString:dateString];        
  NSLog(@"Date/Time is %@", date);

returns:

2011-01-04 16:50:35.369 WA[888:207] Date/Time is 2011-01-04 21:50:35
2011-01-04 16:50:35.370 WA[888:207] Date/Time is 2011-01-04 21:50:35 +0000