Starting a stopwatch is an example of prospective timing: determining the passage of time starting from the present into the future. In contrast, if you walk into a room just in time to see the last grains of sand trickle through the neck of an hourglass, you can deduce something about how much time has elapsed since a past event: an hour ago someone flipped the hourglass over. But unless you flip it over again, the hourglass provides no information about how much time has elapsed since you entered the room. This an example of retrospective timing: estimating the passage of time from some
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