Accessing thenDate Crashes... 'NSInvalidArgumentException', reason: '-[__NSCFString timeIntervalSinceReferenceDate]: unrecognized selector sent to instance 0x7659160'

ViewController.h

#import <UIKit/UIKit.h>

@interface ViewController : UIViewController
@property (nonatomic) NSDate* thenDate;
@property (nonatomic) NSDate* nowDate;
@property (retain, nonatomic) IBOutlet UILabel *theTimer;
@property (retain, nonatomic) IBOutlet UILabel *theLabel;
-(void) updateTimer:(id) sender;
@end

ViewController.m

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController
@synthesize theLabel;
@synthesize theTimer;
@synthesize thenDate;
@synthesize nowDate;

-(void)updateTimer:(id)sender{
    nowDate = [NSDate date];
    
    NSTimeInterval startTime = [nowDate timeIntervalSinceReferenceDate];
    NSTimeInterval endTime = [thenDate timeIntervalSinceReferenceDate];
}

- (void)viewDidLoad
{
    [super viewDidLoad];
    thenDate = [NSDate date];
    nowDate = [NSDate date];
    [NSTimer scheduledTimerWithTimeInterval:1.0f target:self selector:@selector(updateTimer:) userInfo:nil repeats:YES];
    theLabel.text = @"Time";
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

- (void)dealloc {
    [theLabel release];
    [theTimer release];
    [super dealloc];
}
@end