Which calender is correct ? Poornimantha (month ending with poornima) or Amantha (mon

[Mukunda Chaitanya]

Which calender is correct ? Poornimantha (month ending with poornima) or Amantha (month ending with Amavasya) ?

[ameyAtmA]

Namaste

The calendar (panchAng) is a reference framework.

Typically VaishNav follow the PurNimA system.
SmArta and others follow the amAvasyA system.

This is why VaishNavas celebrate KArtik before/around Diwali, and for the others, KArtik comes later.
A Temple with multiple Deities (Shaiva, smArta, VaishNav) will have to follow the amAvasyA framework , I think.

You can do a test by asking various Temple priests.