Without looking up the form when you are claiming the # of days exclusion (Which does not have to be the calendar year by the way) there is an actual schedule you fill out which shows enter dates, exit date, and number of days spent in each country involved. There are special rules on travel days where you cross a border as well. you will need to read up on those to fill out the form correctly. As I recall the 2555 instructions were pretty helpful, but it has been a few years since I completed one.

After seeing sjrcpa's question - My answer is related to the physical presence test I believe, which I think is the only one I have ever used.

And the Schedule I referenced is part III top of page 2