How does USCIS calculate the extended stay outside the U.S. for green card holders?

U.S. permanent residence status is for people who will spend most of their time inside the United States. Also, U.S. permanent residents must file federal income tax returns and must pay all applicable taxes. If you are counting the number of days that you spend inside the U.S. like this, then you should not be a U.S. permanent resident.

Six months without a return.

The calculation is strictly based on the counting of the total days you spent out of the country. The basic counting starts from the day you leave the country and when you come back. For instance, if you spent 180 days, that would translate to six months based on the presumption that every 30 days add up to a month. The calculation is not in any way based on any calendar month of the year such as January or December as you presumed.

The time starts when you receive your green card. You should maintain your intention to permanently reside in the United States and be able to prove such intention, such as having a job, owning property paying taxes, having family members in the United States. While you have your green card, you should not be absent from the United States for more than six months on any given trip abroad.

There is no rule that you can be out for six months a year. The assumption is if you out less than six months, then the U.S. is your home because you spend most of your time here. The question is where do you live. A green card requires you to live here permanently and if you do not, then you need a reason to be out.