Shepherd IS a glorified scheduler It's pretty pluggable, i.e. you have inputs and actions and can define actions for specific combination of input parameters. I will naturally make (and already did) a few 'default' input and action plugins to provide basic functionality - for more complex inputs and actions I'm just trying to keep it as open ended as possible, so interested parties could implement those with minimum effort even if I personally don't have the time or knowledge to do it.

Shepherd technically IS (also) a cron daemon. It does not extend cron. Cron is just a 'unix standard' subset of what Shepherd can do.

You have both stock cron and a more sofisticated alarmd. The 'extra' Shepherd adds is that none of those allow (extensible!) event-based scheduling for parameters other than time.