Using the iOS Notification Center widget, you can start and stop Hours timers no matter where you are on your phone, even from your lock screen.

To set it up, swipe down from the very top of your screen, make sure the Today tab is selected (swipe screen if Recent is displayed), then scroll to the bottom of the screen. Tap the Edit button, find Hours on the "More Widgets" list, then tap the green plus button to add it to the widget. You can then drag the Hours widget to the position you want it to appear in your Notification Center widget list. Then tap Done.

Now whenever you swipe down from the very top of the screen, even on the lock screen, the "Today" tab of the Notification Center widget will show the last Hours timers you have used, and you can tap them to start and stop timers. You can also tap the Hours screen to open Hours right from the widget.