Feature Request: Allow special characters such as L and W

[Phumix]

This is just a thought I had that would be useful:
Would it be possible to allow special characters like L and W in crontab expressions? This would allow the expressions to be a bit more advanced allowing the cron to be set to example last sunday every month at a specific time. Maybe there could be another input box in the execution schedule called "Advanced" where you can type in your own crontab expression manually.