180 days in a row or 180 total within one year to maintain status

[LTP]

Hi, With regard to the 180 days of travel rule...I'm confused on whether the 180 days outside of US is counted in a row, or if it is added up in total over the course of a year.

I'm a green card holder and travel home to canada every 2-3 months for 2-3 weeks. However, I may need to stay longer this time and I need to know if I'm counting days outside of us in total over a year, or just counting from my next entry into canada.

Thanks