Storing Modified Dates - Core Data

Hi all,

I have a core data application which synchronises data with an online  
service. Synchronisation needs to be done by comparing a 'modified  
date' field.

I thought I would be able to control the modified date field by doing  
the following in my base managed object class:

- (void) willSave
{
	NSDate *now = [NSDate date];
	if (false == [[self modifiedDate] isEqualToDate: now])
	{
		[self setPrimitiveValue:now forKey:@"modifiedDate"];
	}
}

This didn't work because the entity is saved whether it has been  
modified or not. So... short of adding a 'isDirty' flag, monitoring  
each setter and allowing for loading of models, is there a good way of  
doing this? I'l all out of ideas...

Thanks!
Trent.