Dr. Lotika Varadarajan has published on the calendrical systems of Nicobar.
One of the interesting calendar notation is the Palm leaf calendar called the Rai Teevla.
The Rai Teevla is calculated on a palm leaf where leaves are knotted and bent variously. The palm leaf calendar can be considered an invitation. The knotted leaves on top indicates the number of days from the date of reciept within which the recipient is expected to reach residence of host. The bent leaves below indicate the days elapsed.
In the following photos, Mr. T. Timothy from Champin demonstates a palm leaf calendar:
