dateByAddingTimeInterval argument syntax

Question

If I send the method dateByAddingTimeInterval: to NSDate, like below:

NSDate *today = [NSDate date]; 
NSDate *tomorrow = [now dateByAddingTimeInterval:24.0];
NSDate *yesterday = [now dateByAddingTimeInterval:-24.0];

NSArray *dates = [NSArray arrayWithObjects: today, tomorrow, yesterday, nil];

NSLog(@"today's date is %@", [dates objectAtIndex:0]);
NSLog(@"yesterday's date was %@", [dates objectAtIndex:2]);

I get this output:

...The first date is 2012-08-30 02:14:19 +0000
...The third date is 2012-08-30 02:13:55 +0000

Which is strange because the third date should have been 2012-08-29

But... if I change the NSDate messages to:

NSDate *today = [NSDate date];
NSDate *tomorrow = [now dateByAddingTimeInterval:24.0 * 60.0 * 60.0];
NSDate *yesterday = [now dateByAddingTimeInterval:-24.0 * 60.0 * 60.0];

Why does adding the * 60.0...

...The first date is 2012-08-30 02:15:25 +0000
...The third date is 2012-08-29 02:15:25 +0000

make the output correct?

Thank you.

Answer 1

NSDate compute time by second.