You've just got a couple of superfluous semi-colons in around the T. Change:
Google should accept that but you'll need to give it time to re-spider your pages again. Give it 24-48 hours and you should see things resolved back in Webmaster tools.
To provide a longer explanation for anyone who's interested; Google expects the date to be in a W3C valid date/time format. Both of these are acceptable:
But if you specify the time then you must also specify the timezone. In this case you're doing so with GMT so that's fine.
As mentioned, you can also use:
Which is actually just a shortcut for the corrected code provided above. It will output the exact same thing.