Let's see -

• Call DL to verify that it was a change and not a delay
• Check online - stat would say DELAYED, not ON TIME for the new time
• Ask the agent at check-in (who crossed out the departure time - I've never seen that happen btw)
• Um, maybe look at the screens throughout the airport!!!
• Go to the Departure Gate earlier than usual boarding time to see what's going on
• etc

Sorry but I wouldn't rely on an automated email to dictate when I would arrive at the airport and decide to wander off for something to eat.