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objective-cioscore-datakey-value-codingnstimeinterval

Difference between dot syntax and valueForKey

Please note that update 3 is probably most relevant

Im setting a NSTimeInterval property of a managed object with an nsdate object using setValue:forKey:

When i attempt to get the value I get weird stuff, at runtime this 

NSLog(@"[managedObject valueForKey:@\"startTime\"] : %@, [NSDate dateWithTimeIntervalSince1970:[managedObject startTime]]: %@",
[managedObject valueForKey:@"startTime"],[NSDate dateWithTimeIntervalSince1970:[managedObject startTime]]);

Returns 

[managedObject valueForKey:@"startTime"] : 2012-07-14 08:13:05 +0000,
[NSDate dateWithTimeIntervalSince1970:[managedObject startTime]]: 1981-07-14 08:13:05 +0000

Update 1
The value returned by [managedObject valueForKey:@"startTime"] is correct. However I would prefer to use [NSDate dateWithTimeIntervalSince1970:[managedObject startTime]] or something similar so that it is more strongly typed.

I believe [managedObject startTime] returns an incorrect value => 363954111.000000 .
However i set it with something like this:

managedObject setValue:1342261311 forKey:@"startTime"

It is worth noting that I am unsure whether this is incorrect because [managedObject valueForKey:@"startTime"] returns a correct NSDate object.

Update 2
I've logged the double values returned by KVC and . syntax. 

managedObject.startTime = 363954111.000000
valueForKey timeIntervalSince1970 = 1342261311.000000

Update 3
Okay, I've set up a test, start time is set like this entity.startTime = [[NSDate dateWithTimeIntervalSince1970:1342261311] timeIntervalSince1970]; and end time is set like this [entity setValue:[NSDate dateWithTimeIntervalSince1970:1342261311] forKey:@"endTime"];

When i write them to log i get this start = 1342261311.000000, end = 363954111.000000

It seems that the NSDate object is being unwrapped incorrectly, has anyone seen this before?

Solution

  • It was a problem with the difference in epochs. NSDate uses Jan 1 2001 as an epoch. So when I was getting the value I was using the unix epoch (1970). That gave me a difference in values.

    When KVC unwraps and wraps NSTimeInterval with a NSDate object it uses the NSDate 2001 epoch.

    So instead of using dateWithTimeIntervalSince1970 I used dateWithTimeIntervalSinceReferenceDate when getting the value.