Calculating the growth rate of a stock over a year with imperfect information I am trying to find the growth rate of a stock over a given year.

Let's say I wanted to find the growth rate from today, June 11, 2015 to June 11, 2014. This is easy enough when you have perfect information and both sets of data exist.

However let's say June 11,2014 stock data does not exist, but June 10th and June 12th do. Which date do I compare to? Do I extend the "year" by a day and go to June 10th, or do I shorten the year and go to June 12th?
I don't think that on yearly returns, a day more or less matters too much. In the unlikely case it actually does, you could interpolate the final day. 

A first approximation would be the average over June 12th and June 11.