standard for % change calculations?

When looking at future index prices, what do they mean by "% change"? Percent change relative to when? Last week? Yesterday afternoon? 23:59? 00:00? Is there a standard for percent change calculations, or does each source calculate it differently?

 

(I should mention, I'm talking about sources like Yahoo, Google, etc)

 

I assume from 15:30PM (Central Time). Futures stop trading from 15:15PM to 15:30PM

 

Based on NY close of RTH for US futures.

 

Depends the exchange.
You can see on the intraday chart.
What's the open price / time and the GMT.

 

Should be based on the prior day's settlement price at the exchange.

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