It's counted by the "date of first entry". The length of stay includes the date of arrival and the date of departure as full days. Thus, if you enter on January 1, the 180th day is June 29. You could re-enter on June 30, which would then be your date of first entry within the new 180-day period.
Here's some general Schengen documentation, though you might find more reader-friendly references on the sites of the consulates for Schengen members.
Schengen agreement