THANK YOU! I actually missed this stamp earlier and couldn't find it.It's a REALLY old post but the "optional area" is the room after you grab all the key tokens to unlock a teleport box. You could easily skip it and head right to the flag if you have a propeller box, but for 100% (including one Green Star) it you should collect all those tokens.
The optional room looks like this
Eventually, you'll see a Spikes that throw stuff. But check the projectiles they throw. They don't break.
So, follow where they go, which is outside
And then you'll see a hidden room with roofs.
You'll need a Propeller Box to get on these roofs, so try not to lose it while you're in that room to either the Goombas or the Spikes.
Anyway the stamp is on the middle roof (the left and right roofs have clocks). I'm on the left roof, but you'll easily see the stamp on the middle one.