Compared to What?

When news on the radio says that a given investment instrument's price is up or down by some stated percentage or monetary amount, but it doesn't say what that's compared to, is there a convention defining this? For example, do they mean a year ago, a day ago, a minute ago, since the last opening or closing, or since some event that is in the news about that instrument, such as a board choosing a new company president? This happens often on Bloomberg radio, but I visited their website and couldn't find any page that explained this.

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