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Create a scheduled script that runs once per day during the night when there are no other scheduled scripts or tasks.
The supplied time is in UTC so make sure that LM has correct timezone set for the automatic conversion to work.
For testing purposes you can comment out os.sleep. It is needed so the script is not triggered again if the time is shifted.

Code:

os.sleep(30) require('serial') port = serial.open('/dev/ttyACM0') if not port then   log('port not found')   return end port:flush() function settime(line)   local parts = line:split(',')   if parts[1] ~= '$GPRMC' then     return   end   local hour, min, sec = string.match(parts[2] or '', '^(%d%d)(%d%d)(%d%d)%.')   local day, month, year = string.match(parts[10] or '', '^(%d%d)(%d%d)(%d%d)$')   if not hour or not day then     return   end   local cmd = string.format('date -u -s "20%s-%s-%s %s:%s:%s"; hwclock -w',     year, month, day, hour, min, sec)   os.execute(cmd)   return true end buf = {} lines = 0 while true do   char = port:read(1, 1)   if char then     if char == '\r' or char == '\n' then       if #buf > 0 then         line = table.concat(buf)         lines = lines + 1         if settime(line) then           log('time updated')           break         elseif lines > 20 then           log('could not update time')           break         end         buf = {}       end     else       buf[ #buf + 1 ] = char     end   end end