How to know if crontab implements @reboot?

according to crontab.guru, many functions like @reboot, @daily, @annually ... are not standard. Is there any way to know the one implemented on my system (Kubuntu 21.04) ? I have tried the man-page, but have no result.

Thank you

The supported "special strings" for time specification are listed in man 5 crontab:

   Instead  of the first five fields, one of eight special strings may ap‐
   pear:

          string         meaning
          ------         -------
          @reboot        Run once, at startup.
          @yearly        Run once a year, "0 0 1 1 *".
          @annually      (same as @yearly)
          @monthly       Run once a month, "0 0 1 * *".
          @weekly        Run once a week, "0 0 * * 0".
          @daily         Run once a day, "0 0 * * *".
          @midnight      (same as @daily)
          @hourly        Run once an hour, "0 * * * *".

   Please note that startup, as far as @reboot is concerned, is  the  time
   when  the cron(8) daemon startup.  In particular, it may be before some
   system daemons, or other facilities, were startup.  This is due to  the
   boot order sequence of the machine.

If you don't believe the documentation for your system, then you may download the source (ex. apt-get source cron) and check the entry.c file:

cron-3.0pl1$ grep '!strcmp' entry.c
                if (!strcmp("reboot", cmd)) {
                } else if (!strcmp("yearly", cmd) || !strcmp("annually", cmd)){
                } else if (!strcmp("monthly", cmd)) {
                } else if (!strcmp("weekly", cmd)) {
                } else if (!strcmp("daily", cmd) || !strcmp("midnight", cmd)) {
                } else if (!strcmp("hourly", cmd)) {