Cron is one of those things I’ve been using for twenty-odd years and still occasionally stare at for thirty seconds before committing. Is */15 9-17 * * 1-5 what I think it is? Does 0 0 * * 0 fire at midnight Sunday or Monday? And does that depend on the box’s timezone or UTC? Most of the time you can squint and reason your way through it, but “most of the time” is exactly the kind of confidence level you don’t want when the cron in question is the nightly DB backup.
The cron expression explainer on the Toolbelt is meant to take the squinting out of the loop. You paste an expression, and it gives you a plain-English summary, a per-field breakdown (so you can see which field is doing the weird thing), and the next handful of run times in a timezone of your choice. The schedule preview is the bit I use most - it’s much faster to confirm “yes, that fires at the times I expect” than to reason about the cardinality of */15 from first principles.
It handles the standard 5-field syntax plus the common shortcuts (@daily, @weekly, etc.). No more “I think this is right, let’s see in production”.