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You can create a store whose value is based on the value of one or more other stores with derived. Building on our previous example, we can create a store that derives the time the page has been open:

export const elapsed = derived(time, ($time) => Math.round(($time - start) / 1000));

It's possible to derive a store from multiple inputs, and to explicitly set a value instead of returning it (which is useful for deriving values asynchronously). Consult the API reference for more information.