Maintaining the residency as a green card holder

[basia]

Hi there, I have a question. As a green card holder since 9/16/2004 and an American citizen's spouse, I have been traveling a lot back and forth to my country of origin and the other countries overseas, but never longer then 6 month in a row. I was also concerned not to spend less than 180 days a year out of the US. I am only not sure, what counts as a year - a calender year or the period of time between 9/16 and 9/15 each year?. FYI, I have spent more then 18 months in the US during the last three years, as required.
I would appreciate your suggestions.

[knowledgeable]

Actual time period

It's not the calendar year, but the actual time period from when you got the permanent residency to when you will apply for the US citizenship.

Take care.