You can store the NSDate in a variable. Then in viewWillDisappear you save the date:
-(void)viewWillDisappear:(BOOL)animated {
NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
[defaults setObject:lastPickedDate forKey:@"lastDate"];
}
Then, when you get back to this screen you can verify if there's a date saved and restore it in viewDidLoad or viewDidAppear.
-(void)viewDidLoad {
//another code
NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
NSDate *date = [defaults objectForKey:@"lastDate"];
if(date) {
_datePicker.date = date;
}
}