The SG-2220 does not have an RTC clock battery so if it's been off for some time it may revert to the initial time/date.
If you do not have at least one NTP server defined by IP and you have DNSSec enabled in Unbound and no other DNS servers set then you have a chicken/egg situation. The firewall cannot recolve any time servers because DNS doesn't work when the clock is wrong!
Setting either a fixed NTP server or an alternative DNS server will prevent that.
Steve