NSDate format and value strange behavior

问题

I have the following code:

NSDateFormatter * df = [[NSDateFormatter alloc] init];
    [df setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
    [df setTimeZone:[NSTimeZone systemTimeZone]];
    [df setFormatterBehavior:NSDateFormatterBehaviorDefault];

myres.start_date = [df dateFromString:[NSString stringWithFormat:@"%@ %@", [self.data objectForKey:@"date"], [self.data objectForKey:@"start_time"]]];
    NSLog(@"Start date is %@", [NSString stringWithFormat:@"%@ %@", [self.data objectForKey:@"date"], [self.data objectForKey:@"start_time"]]);
    NSLog(@"Start date from NSDate is %@", myres.start_date);
    myres.end_date = [df dateFromString:[NSString stringWithFormat:@"%@ %@", [self.data objectForKey:@"date"], [self.data objectForKey:@"end_time"]]];
    NSLog(@"End date is %@", [NSString stringWithFormat:@"%@ %@", [self.data objectForKey:@"date"], [self.data objectForKey:@"end_time"]]);
    NSLog(@"End date from NSDate is %@", myres.end_date);

and the result I am getting is:

Start date is 2011-03-31 9:00:00
Start date from NSDate is 2011-03-31 16:00:00 +0000
End date is 2011-03-31 10:00:00
End date from NSDate is 2011-03-31 17:00:00 +0000

why is the result different? I would like to have it so that when I call myres.start_date description, it gives me the same data as the string.

回答1:

The result is different because you are printing your NSDate instances using the generic description method, which doesn't match the output format you expect.

If you used [df stringFromDate:myres.start_date] you should get the same output.

Keep in mind that NSDate stores a representation of time, and there are many ways to print time and description's way of printing time is fixed.

The only way to have description of NSDate return a string in the format you want is to override it with a category or use a subclass.

You seem to have a fundamental misunderstanding of how time is stored and displayed: your NSDate is storing the time parsed by NSDateFormatter, and it is in fact returning out the same data in its description method. It is just displaying it in a different form.

回答2:

The date formatter takes the time zone into account. When you use the date directly, you just get it's description, which is apparently zulu time.

来源：https://stackoverflow.com/questions/5509252/nsdate-format-and-value-strange-behavior

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