How does Google calculate population by month and county?

[pena...@gmail.com]

How or where does Google get the monthly estimates for the population? For instance, Google lists Los Angeles County population for the last 20 or 30 years by month. I have never seen this in the Census data. Can someone tell me  "how" and "where" do they get the data by month? How accurate are they? I asked CA the Financial Department since they do population counts but they only do them twice a year.

 Thanks