The time they give you is the time your bus is scheduled to leave. You need to be at the platform, completely ready to go, 15 minutes before that. You also need to allow time getting anything you've left with bell services and a last minute bathroom stop (trust me, someone will need it.)
I would be heading out of the gates no later than an hour and 45 minutes before your DME departure. Yes, you may have some time to kill, but it's better than scrambling to get a taxi at the last minute because an unexpected delay caused you to miss your bus.