The Buddhist calendar is used on mainland southeast Asia in the countries of Cambodia, Laos, Thailand, Myanmar (formerly Burma) and Sri Lanka in several related forms. It is a lunisolar calendar having months that are alternately 29 and 30 days, with an intercalated day and a 30-day month added at regular intervals. All of its forms are based on the original third century Surya Siddhanta, not its modern form (both forms are used by the various Hindu calendars). The month names are Sanskrit (except in old Burmese): The old Burmese month names were: