The planet rotates, John. It's a measure of the rotation from an initial point. It's down to the hour (or half hour depending) for simplicity's sake, all kinds of things do calculations down to the second or more for accuracy. Linear time constrains humans because we can't experience it any other way so even our technology assumes linear time in the physics, as the Godzilla of Tech you might be familiar with something like how server locations are decided.
edit: Interesting, they're just a social construct, John:
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