When “the time is off too much” certificates will be declared invalid. Surveys have measured how frequent this is for the open web (which is what we mostly care about at Let’s Encrypt). I will see if I can find such a survey to link here. But IIRC the main takeaway is that most clients have the right date and are only out by e.g. one hour for the wrong time zone or for too much / not enough daylight savings. For Google this probably means certificates shouldn’t be used for the first 90 minutes after they’re issued, for most of us it barely matters.
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