I’ve just come from my human rogue on Emerald Dream to my BE Paladin on Moonguard, and my calendar button has definitely disappeared. I even took screenshots to send to a friend just to prove I’m not crazy.
On my other toon (toons? is this the only one with the issue? probably), the calendar button is right next to the zone name and right above the addon counter, right below the time on my TitanPanel.
On this paladin, that calendar button is missing. /calendar still works, but I only figured that out after searching for this issue on Google…and finding the last four times this has happened in WoW’s life.