I thought that it was mentioned that in one of the updates there was going to be a Cron string generator. I may not be remembering right as I don’t find it anywhere.
There was, but it's in Rule Machine when you create a "custom" periodic trigger as a cron string rather than one of the pre-defined intervals.
That being said (guessing based on where you posted this), it's the same format you'd need for a cron string in a custom app if you're using that instead of runEvery<X>() or whatnot, so it could serve another purpose regardless of its intent. 