The Indian national calendar is a solar calendar, which means that it is based on the tropical year. It does not support lunar months, which are based on the synodic month, which is the time it takes for the moon to return to the same phase as seen from Earth. However, there is a proposal on GitHub at Lunar Month support in Intl.DateTimeFormat() · Issue #538 · tc39/proposals · GitHub that would add support for lunar months to the Indian national calendar.