This site lists PE based on trailing 12 months earnings as 27.75: https://www.multpl.com/s-p-500-pe-ratio

So PE of 23 is probably based on projected "forward earnings" which isn't really that useful. The most common time period for both is 12 months and the projections are usually coming from a combination of guidance from each company and analysts at banks and investment companies. Obviously there's a large margin of error in these estimates.