- (id) init
{
	self = [super init];
	if (self != nil) {
		// If past expiration date, then pop up a dialog that says 
		// the user needs to download a new version, quit the app, and
		// load the website for new downloads
		
		NSDate *now = [NSDate date];
		NSDate *expire = [NSDate dateWithNaturalLanguageString:@"4:30pm May 19, 2009"];
		
		if ([now timeIntervalSinceReferenceDate] > [expire timeIntervalSinceReferenceDate]){
			NSLog(@"Beta Expired");
			if (NSRunAlertPanel(@"This beta of App has expired.", 
					@"Please download a new version from\nhttp://appwebsite.com.", 
					@"Go to the Download Page", @"Maybe Later", nil)) {
				[self openURL:@"http://appwebsite.com/"];
			}
			[[NSApplication sharedApplication] terminate:self];
		}
	}
	return self;
}

- (void)openURL:(NSString *)url {
	// open URL in browser
	[[NSWorkspace sharedWorkspace] openURL: [NSURL URLWithString:url]];	
}

- (id) init
{
	self = [super init];
	if (self != nil) {
		// If past expiration date, then pop up a dialog that says 
		// the user needs to download a new version, quit the app, and
		// load the website for new downloads
		
		NSDate *now = [NSDate date];
		NSDate *expire = [NSDate dateWithNaturalLanguageString:@"4:30pm May 19, 2009"];
		
		if ([now timeIntervalSinceReferenceDate] > [expire timeIntervalSinceReferenceDate]){
			NSLog(@"Beta Expired");
			if (
				NSRunAlertPanel(@"This beta of App has expired.", @"Please download a new version from\nhttp://appwebsite.com.", @"Go to the Download Page", @"Maybe Later", nil)
				
				) {
				[self openURL:@"http://appwebsite.com/"];
			}
			[[NSApplication sharedApplication] terminate:self];
		}
	}
	return self;
}

- (void)openURL:(NSString *)url {
	// open URL in browser
	[[NSWorkspace sharedWorkspace] openURL: [NSURL URLWithString:url]];	
}

Implement this in YourAppController delegate class to check for an expiration date when the app is first initialized.

Application Expiration

Objective C