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system
rate of return of stock index
user
the return from one day to the next is based on the day's closing price.  to be clear - opening prices can be quite different from the prior day close. in your example, they are pretty close, but this is not always the case. just pull a larger data set to observe this.  the above aside, dividends are not reflected in the index, so, after a dividend has occurred, you'd need to account for this if you are looking for true total return. in 2011, the s&p closed at 1257.60 vs a 2010 year end 1257.64. the return, however was 2.11%, not zero, after accounting for the dividends. to me, articles that suggest the yearly return was zero are inaccurate and misleading.
assistant