systemd was created to solve the problems of a directory full of shell scripts. A single shell script has completely different problems. And traditional init uses inittab, which is not /etc/init.d, and works more like runit.

runit's approach is to just keep trying to start the shell script every 2 seconds until it works. One of those worse–is–better ideas. You can check for arbitrary conditions and error–exit, and it will keep trying. If you need the time synced you can just make your script fail if the time is not synced.