Answer

In the traditional Indian lunar calendar, Amavasyant and Purnimant are two ways of counting a lunar month.

• Amavasyant month: The month ends on Amavasya, the new moon day.
• Purnimant month: The month ends on Purnima, the full moon day.

The main difference is where the lunar month is considered to end and begin.

Explanation

A lunar month has two fortnights:

• Shukla Paksha – bright half, from new moon toward full moon
• Krishna Paksha – dark half, from full moon toward new moon

Amavasyant System

In the Amavasyant system:

• The month begins after Amavasya.
• It starts with Shukla Paksha.
• It ends on the next Amavasya.

So the order is:

Shukla Paksha → Purnima → Krishna Paksha → Amavasya

Purnimant System

In the Purnimant system:

• The month begins after Purnima.
• It starts with Krishna Paksha.
• It ends on the next Purnima.

So the order is:

Krishna Paksha → Amavasya → Shukla Paksha → Purnima

Important Point

The actual moon phases are the same. Only the naming/counting of the month changes.

For example, a particular Shukla Paksha date usually has the same month name in both systems, but the Krishna Paksha date may be assigned to different month names depending on whether the calendar follows Amavasyant or Purnimant reckoning.

In Jain Context

Jain calendars also use lunar months for observing:

• Paryushan / Das Lakshan
• Mahavir Janma Kalyanak
• Ayambil Oli
• Pakshi Pratikraman
• Purnima and Amavasya observances

Because different regions may follow different calendar traditions, festival dates are usually checked according to the accepted Jain calendar or Panchang of that tradition and region.

Takeaway

Amavasyant means the lunar month ends on Amavasya. Purnimant means the lunar month ends on Purnima.

The moon cycle is the same; only the method of naming the lunar month is different.