2 NSDates that should be equal aren't?

Question

I'm using the JSON library from Stig Brautaset(http://code.google.com/p/json-framework) and I need to serialize an NSDate. I was considering converting it into a string before JSONifying it, however, I ran into this weird behavior:

Why aren't these NSDates considered equal?

NSDate *d = [[NSDate alloc] init];
NSDate *dd = [NSDate dateWithString:[d description]];

NSLog(@"%@", d);
NSLog(@"%@", dd);
if( [d isEqualToDate:dd] ){
    NSLog(@"Yay!");
}

Answer 1

When you describe the original date object you lose some sub-second precision from the original object — in other words, -description shaves off fractional seconds, and returns

A string representation of the receiver in the international format YYYY-MM-DD HH:MM:SS ±HHMM, where ±HHMM represents the time zone offset in hours and minutes from GMT

When you create a new date object based on the description, you get it in whole seconds because the string is only precise to a whole second. So -isEqualToDate: returns NO because there is a difference of a fraction of a second between your two date objects, which it's sensitive to.

This method detects sub-second differences between dates. If you want to compare dates with a less fine granularity, use timeIntervalSinceDate: to compare the two dates.

So you'd do something like this instead (NSTimeInterval measures in seconds):

if ([d timeIntervalSinceDate:dd] == 0) {
    NSLog(@"Yay!");
}

Answer 2

isEqualToDate detects subseconds differences between dates, but the description method does not include subseconds.

Answer 3

Because they're not equivalent:

NSDate *d = [NSDate date];
NSDate *dd = [NSDate dateWithString:[d description]];
NSLog(@"%f", [d timeIntervalSinceReferenceDate]);
NSLog(@"%f", [dd timeIntervalSinceReferenceDate]);

Produces:

2011-04-28 11:58:11.873 EmptyFoundation[508:903] 325709891.867788
2011-04-28 11:58:11.874 EmptyFoundation[508:903] 325709891.000000

In other words, the +dateWithString: method does not maintain sub-second precision.