My Mayan Aunt has found the other half of the calender.
It seems that the reason the calender finished on the 21st was because the other half got lost down the back of a stone couch all those years ago. My Aunt has just realised, it's in her handbag having been retrieved when the the Mayans moved on and sold their stone furniture to an ancestor of my Aunt, who passed on the calender they retrieved from it.
You can all stop worrying now, because this calender goes to the year 3752, March 16th.
Hope this helps ease the nerves of those continually feeling like they need a poo.