As noted by More Fiber
there is no hardware time of day clock in the AT and the time is solely maintained by the running kernel. Whenever the AT restarts, the time will default back to a hard-coded value until it is able to determine the proper current time via NTP. The reason it restarts is not particularly relevent - either a reboot or a power cycle will have the same effect. Seeing the default time and date in the logs right after any form of startup is perfectly normal.