Needs to be moved to /sbin/rEboot, plays together with previous reboot gist

rEboot

#!/bin/bash
if [ -f /home/pi/last_reboot ]; then 
  start=`date +%s`
  reboot=`cat /home/pi/last_reboot`
  rm /home/pi/last_reboot

  # wait for NTP update
  last=0
  now=$last

  # skip waiting in case DS3231 RTC is active via i2c-rtc dtoverlay
  if [ -f /usr/sbin/i2cdetect ]; then
    if [ "`i2cdetect -y 1 0x68 0x68 | grep UU`" != "" ]; then
      last=$(($now-2))
    fi
  fi

  until [ $(($now-$last)) -gt 1 ]; do
    last=`date +%s`
    sleep 0.5
    now=`date +%s`
  done
  
  xmessage $((($start-$reboot)+($now-$last-1))) seconds for last reboot
fi