operation stops and resets between storyboards - ios

Question

My main storyboard has a stopwatch. I can start the watch, and then navigate to another storyboard. When I return to the main stopwatch storyboard it has stopped and reset to zero. Please help me figure out how to keep the stopwatch going forever until I press STOP. Thanks
All storyboards have same controller
here is the .h
#import <UIKit/UIKit.h>
#interface MainViewController : UIViewController{
UILabel *lbl;
NSTimer *stopTimer;
NSDate *startDate;
BOOL running;
}
#property (weak, nonatomic) IBOutlet UIBarButtonItem *sidebarButton;
#property (strong,nonatomic) IBOutlet UILabel *lbl;
-(IBAction)startPressed:(id)sender;
-(IBAction)resetPressed:(id)sender;
-(void)updateTimer;
#end
here is the .m
#import "MainViewController.h"
#import "SWRevealViewController.h"
#interface MainViewController ()
#end
#implementation MainViewController
#synthesize lbl;
- (void)viewDidLoad
//stopwatch open
{
[super viewDidLoad];
lbl.text = #"00.00.00.000";
running = FALSE;
startDate = [NSDate date];
//stopwatch close
[super viewDidLoad];
self.title = #"therapyTools";
// Change button color
_sidebarButton.tintColor = [UIColor colorWithWhite:0.1f alpha:0.9f];
// Set the side bar button action. When it's tapped, it'll show up the sidebar.
_sidebarButton.target = self.revealViewController;
_sidebarButton.action = #selector(revealToggle:);
// Set the gesture
[self.view addGestureRecognizer:self.revealViewController.panGestureRecognizer];
}
- (void)didReceiveMemoryWarning
{
[super didReceiveMemoryWarning];
// Dispose of any resources that can be recreated.
}
//stopwatch open
-(IBAction)startPressed:(id)sender{
if(!running){
running = TRUE;
[sender setTitle:#"STOP" forState:UIControlStateNormal];
if (stopTimer == nil) {
stopTimer = [NSTimer scheduledTimerWithTimeInterval:1.0/10.0
target:self
selector:#selector(updateTimer)
userInfo:nil
repeats:YES];
}
}else{
running = FALSE;
[sender setTitle:#"START" forState:UIControlStateNormal];
[stopTimer invalidate];
stopTimer = nil;
}
}
-(void)updateTimer{
NSDate *currentDate = [NSDate date];
NSTimeInterval timeInterval = [currentDate timeIntervalSinceDate:startDate];
NSDate *timerDate = [NSDate dateWithTimeIntervalSince1970:timeInterval];
NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateFormat:#"HH:mm:ss.SSS"];
[dateFormatter setTimeZone:[NSTimeZone timeZoneForSecondsFromGMT:0.0]];
NSString *timeString=[dateFormatter stringFromDate:timerDate];
lbl.text = timeString;
}
-(IBAction)resetPressed:(id)sender{
[stopTimer invalidate];
stopTimer = nil;
startDate = [NSDate date];
lbl.text = #"00.00.00.000";
running = FALSE;
}
//stopwatch close
#end