If it happens during system update with APT, it means that your system time & date are not correct. Normally it happens if you don't have real time clock (RTC) or other time source (e.g. GPS) connected to your raspberry pi. If you start updating just after boot, before raspberry picks up the time from the network, it will access astroberry.io site using local time in 1970 or so. it looooong before astroberry.io site certificate was issued so you get error. It will not appear if you retry after a few minutes, when your system time is correct.

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