The Jerrold spreadsheet is very good, but for my needs it was over-featured, to coin a phrase. I needed something simpler.
Adapting from a free Excel template I found online, I put together a much simpler Gantt-ish spreadsheet. It doesn't do all the cool stuff Jerrold's does, but for my purposes it's useful to get a quick overview of my basic scheduling needs.
It also works with concrete dates, whereas Jerrold's models time more abstractly.
I'm sharing a link to an iCloud Numbers page where anyone interested can download it and see if it's what they want:
https://www.icloud.com/numbers/0yZBXnEpZqltSJMkJdME-_Enw#Simple_Gantt_With_Dates
It's clunky in many ways, if anyone's interested in modifying it to make it more robust, go ahead! I made a backup, so if it gets borked I can restore it easily.