I think the office is open from 8 to 3 or 3:30, so you should be okay so long as you arrive no later than you plan. We've always gotten our extension stamp in the morning, but I don't think there's any time of day they don't perform that service. It's very possible that they close for lunch, but I am not sure about that.
The office is directly across from the airstrip above Scotia Bank. You'll go to the Immigration office first for them to check your passport (last door on the south end of the building), they'll send you back up a couple doors to the clerk's office to pay, then you'll go back to the Immigration office for the stamp.