compare two dates in objective-c - objective-c

Question

I am sure this question came up before I am pulling my hair out. I have two dates - one from an Object on Parse.com and the other one local. I try to determine whether the remote object has been updated so that I can trigger actions locally.
When looking at the NSDate of both objects they seem identical but a comparison reveals that the remote object is newer - when checking the time internal (since1970) it becomes obvious that there is a difference but why? When I first created the local object all I did was
localObject.updatedAt = remoteObject.updatedAt //both NSDate
But when looking closer I get this:
Local Time Interval: 1411175940.000000
Local Time: 2014-09-20 01:19:00 +0000
Remote Time Interval: 1411175940.168000
Remote Time: 2014-09-20 01:19:00 +0000
Does anyone have an idea why that is and whether I can ignore this detail? Does iOS round up or something?
Adding more code:
#property (strong, nonatomic) NSDate *date;
...
PFQuery *query = [PFObject query];
[query whereKey:#"Product" equalTo:#"123456"]
[query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {
if (!error)
{
self.date = objects[0].updatedAt;
NSTimeInterval localTime = [self.date timeIntervalSince1970];
NSTimeInterval remoteTime = [objects[0].updatedAt timeIntervalSince1970];
NSLog(#"Local Time Interval: %f", localTime);
NSLog(#"Local Time: %#", self.date);
NSLog(#"Remote Time Interval: %f", remoteTime);
NSLog(#"Remote Time: %#", objects[0].updatedAt);
}
else
{
NSLog(#"Error with query");
}
}];
That results in the console output above - and I don't understand why these dates are different.

Answer 1

I cannot explain why there is a difference, but the important thing to understand is that there can be a difference and that when comparing dates you have to use a tolerance value.
The Apple Date and Time Programming Guide has an example of how to compare two dates within a given tolerance:
To compare dates, you can use the isEqualToDate:, compare:,
laterDate:, and earlierDate: methods. These methods perform exact
comparisons, which means they detect sub-second differences between
dates. You may want to compare dates with a less fine granularity. For
example, you may want to consider two dates equal if they are within a
minute of each other. If this is the case, use timeIntervalSinceDate:
to compare the two dates. The following code fragment shows how to use
timeIntervalSinceDate: to see if two dates are within one minute (60
seconds) of each other.
if (fabs([date2 timeIntervalSinceDate:date1]) < 60) ...
It's up to you decide on the tolerance value, but something like 0.5 seconds seems reasonable:
+ (BOOL)date:(NSDate *)date1
equalsDate:(NSDate *)date2
{
return fabs([date2 timeIntervalSinceDate:date1]) < 0.5;
}

Answer 2

Parse stores dates as iso8601 format. This makes things very complex as Apple does not manage the format well. While the idea of the standard is awesome, until everyone plays by the same rules, anarchy rules..
I convert everything inbound from parse into usable format before attempting anything on their date time values..
Drop this into a library somewhere, and save yourself tons of headaches. This took weeks of searching and scratching to overcome.
+ (NSDate *)convertParseDate:(NSDate *)sourceDate {
NSDateFormatter *dateFormatter = [NSDateFormatter new];
NSString *input = (NSString *)sourceDate;
dateFormatter.dateFormat = #"yyyy-MM-dd'T'HH:mm:ss.SSS'Z'";
// Always use this locale when parsing fixed format date strings
NSLocale* posix = [[NSLocale alloc] initWithLocaleIdentifier:#"en_US_POSIX"];
dateFormatter.locale = posix;
NSDate *convertedDate = [dateFormatter dateFromString:input];
assert(convertedDate != nil);
return convertedDate;
}