[rnai]

Normally this is 12 month trailing PE.

"The PE ratio of the S&P 500 divides the index (current market price) by the reported earnings of the trailing twelve months."

So it's current total market value (add the market cap of all 500 companies in the index),

And divide by the total past 12 months earnings of the same 500 companies.

If that doesn't answer your question, please let me know.