How does Google Analytics calculate the number of Unique Visitors for any arbitrary timespan?

Note that Unique Visitors is not a value you can sum. If I 开发者_Go百科had 10 unique visitors on Sunday and 15 on Monday it doesnt mean I had 25 uniques on Sun-Mon...

The answer is that GA does not do this (i.e., calculate Unique Visitors for an arbitrary time span)--Unique Visitor counts are specific to a given time period.

Put another way, the answer--which is easier to see when you retrieve data via the API and your call includes a data range--is that the Unique Visitor count is tied to the time period specified in your query (API call or GA Browser view).

For instance, for one of our Sites, GA reports 2,316,674 Unique Visitors for 2009; if i sum the UV counts for each of the 12 months then i get 2,844,912.

This view is reiterated in the GA Forums, and in Blog posts on the subject, e.g., in the excellent Lunametrics Blog.