Just to get you started...
You would create a reference date like this:
NSCalendar *gregorian = [[NSCalendar alloc]
initWithCalendarIdentifier:NSGregorianCalendar];
NSDateComponents *dateComponents = [[NSDateComponents alloc] init];
dateComponents.year = 2012;
dateComponents.month = 1;
dateComponents.day = 1;
dateComponents.hour = 0;
dateComponents.minute = 0;
dateComponents.second = 0;
NSDate *referenceDate = [gregorian dateFromComponents: dateComponents];
Current date can be obtained like this:
NSDate *now = [NSDate date];
Time interval is in seconds, so...
NSTimeInterval interval = [now timeIntervalSinceDate:referenceDate];
NSLog (@"reference date was %.0f seconds ago", interval);
I'm sure you'll be able to figure it out on yourself from here on...
This answer might help you out.