Amanta and purnimanta lunar months
A lunar month is counted in two main ways in India: the amanta system ends the month at the new Moon, and the purnimanta system ends it at the full Moon. The two give the same days, but the names of the months differ by half a month.
What it is
In the amanta system the month runs from the day after one Amavasya to the next Amavasya, and it is common in much of western and southern India. In the purnimanta system the month runs from the day after one Purnima to the next, and it is common in northern India.
How it is worked out or read
The tithi numbering is the same in both systems. Only the start of the month and the month's name differ, so the same Krishna Paksha is in the same place in each.
Where it appears on this site
The panchang shows the paksha, which is the same in both systems. The adhik maas article explains extra months.
Two ways to count the month
In India the lunar month is counted in two main ways. The amanta system ends the month at the new moon, and the purnimanta system ends it at the full moon. The two give the same days, but the month's name and the start of the month differ by about half a month.
Regional use
The amanta system is common in much of western and southern India, while purnimanta is common in northern India. Both appear in printed calendars, and a reader should check which system a given calendar uses before comparing dates.
What stays the same
The tithis are numbered the same way in both systems, and the paksha is the same. Only the start of the month and its name differ. A festival tied to a tithi falls on the same day in both systems, but the month in which it is named may differ.
Choosing a system for a region
A family that moves between regions may find that different relatives observe festivals by different month systems. The easiest way to avoid confusion is to agree on one calendar for the family's own observances and to note the system that others use. Knowing the difference allows a person to explain why a festival appears in different months in different places without suspecting an error.
The systems also matter for dating historical documents, where the month name and the start of the month must be known to convert a date correctly. A careful reader checks the system before converting any old record.
Why the months are named as they are
The month names follow the Sun's position at the time of the month's festivals, and the names are used in both systems, though the timing of their start differs by about a fortnight. A reader who understands the naming can see why the same name appears in one system a fortnight earlier than in the other, and why the festivals attached to that name occur at the same tithi in both.