无法比较两个NSDate

Question

Confused here... I have two dates which I NSLog to console and get: dates are 2011-03-30 13:33:57 +0000 - 2011-03-28 13:33:57 +0000

One date is clearly later than the other... However, in the code below it doesn't matter if I use [dateUSERPREFS laterDate:dateXML] or [dateUSERPREFS earlierDate:dateXML] I get "we get first IF" displaying in the console?

Any ideas? Thanks,

NSDate *dateXML = [df dateFromString:last_modifiedXML];
NSDate *dateUSERPREFS = [df dateFromString:last_modifiedUSERPREFS];

NSLog(@"dates are %@ - %@", dateXML, dateUSERPREFS);

if ([dateUSERPREFS laterDate:dateXML]) {        
    NSLog(@"we get first IF");
}

Answer 1

[aDate laterDate:anotherDate] returns an NSDate , not a BOOL . Specifically, it returns the later of the two dates.

You want to use compare: instead:

NSComparisonResult comparisonResult = [dateUSERPREFS compare:dateXML];
if (comparisonResult == NSOrderedAscending) {
    // case where dateUSERPREFS is before dateXML
} else if (comparisonResult == NSOrderedSame) {
    // both dates are the same
} else if (comparisonResult == NSOrderedDescending) {
    // this could have just been a plain else; dateUSERPrefs after dateXML
}

Alternatively, you could use [dateUSERPREFS timeIntervalSinceDate:dateXML] which gives you the (signed) number of seconds from dateXML to dateUSERPREFS .

Answer 2

In the documentation, Apple says that laterDate returns the later of the two dates, not a BOOL statement.

I'd do the following: subtract one date from the other and check if the value is more or less than zero. That'd tell you which is larger:

if ([dateXML timeIntervalSinceDate:dateUSERPREFS] > 0) {
    //Now you know that dateXML is larger than dateUSERPREFS
}

Answer 3

-[NSDate laterDate:] returns an NSDate * not a BOOL , and this won't be Null , so the condition will always be true.

Try -[NSDate compare:] which has the signature:

- (NSComparisonResult)compare:(NSDate *)anotherDate

You can then compare the NSComparisonResult in your if condition, which is I believe what you are trying to achieve with the code in your question.

(Of course, you could continue to use laterDate: and just compare equality with the original object, but I feel compare: makes for more intuitive logic)