The Tibetan calendar is based on the Kalachakra Tantra and like other tantras it is ambiguous, even encrypted, and stands in need of interpretation. The commentators on the Kalachakra Tantra which bothered to comment on the astronomy gave different interpretations, hence we have different calendars. Since Kamstang Kagyu follows the Karmapa, they are going to give a lot of weight to the Karmapa's interpretation of the Kalachakra and use his calendar.
Having obtained the supreme freedoms
and conjunctions of the precious human existence,
endowed with faith, energy, and intelligence,
Having attended on a worthy spiritual friend
and received the pith of the holy instructions,
May we practice these properly, just as we have received them,
without obstacle or interruption.
In all our lives, may we practice and enjoy the holy dharma.