I have fixed it by creating this method
-(void)addToDiary{
UIStoryboard *storyboard = [UIStoryboard storyboardWithName:@"MainStoryboard" bundle:nil];
AddToDiaryViewController *yourViewController = (AddToDiaryViewController *)[storyboard instantiateViewControllerWithIdentifier:@"AddToDiary"];
NSString *dateSelected = [NSString stringWithFormat:@"Date Selected: %@",myTimeZoneDay];
yourViewController.dateString = dateSelected;
[self.navigationController pushViewController:yourViewController animated:YES];
}
and then calling [self addToDiary] in didSelectDate.
Hope this helps someone