The National Calendar
The Gregorian calendar is Bhārata's primary calendar despite the existence of various indigenous calendars, of which, one was even adopted as the Indian National Calendar. Although the official usage of this calendar started on 22 March 1957 or 1 Chaitra 1879, Saka Era, it still lies unused in the civil domain even after almost 60 years.
The use of a single global calendar helps a lot in global interactions. However, to achieve this, the Indian national calendar need not be demoted. Instead, a calendar converter can be used wherever necessary much like we use a time converter wherever necessary. Everyone in the world need not follow a single calendar to function in cohesion much like everyone does not follow a single time zone to function in cohesion.
To be honest, there aren't any intrinsic advantages to using an indigenous calendar. Whatever contrasts from the disadvantages of a foreign calendar — such as inconsistent dates of culturally relevant events and festivals — appear to be the advantage of using indigenous calendar. Moreover, indigenous calendars are cultural elements that we should practice, especially if there is no practical reason not to.
This was about the civil domain. Now, in the cultural domain, ethnicities who have their own calendars such as the Malayāḷam calendar for the Malayāḷi and the Tamil calendar for the Tamillians must use their own calendars. In fact, they (all lunar calendar and all solar calendars) are different only in their names.