Or just have two cron jobs:

    * * * * *    user               dothing
    * * * * *    user   sleep 30 && dothing

them you don't need to muck about noting start time if `dothing` could take a significantly variable number of seconds to complete. Many moons ago I did something similar for a task that I wanted running every 10 seconds (it was a quick hack taking a few readings and dumping the data for me to later graph).

If the task might sometimes take 30 seconds to run you have extra faf debouncing it to avoid race conditions, with this solution or yours, which IIRC systemd timers will deal with (by being configurable to not run the next instance if the previous has not yet completed).