Delegating Zimbra calendar TO Google Calendar

Once upon a time, several years ago, it was possible to subscribe to my Zimbra calendar from my Google Calendar so I could see my Zimbra (work) events alongside my home events. It never worked quite right: if I updated an event in Zimbra it didn’t update in Google, and if I deleted certain (but not all!) events in Zimbra they still showed up on Google, but it was better than nothing.

In late 2022 I tried deleting the subscription and recreating it, but then it stopped working entirely. I got the ICS link from the Preferences → Sharing → Folders Shared By Me → Share A Folder option and pasted it into Google; Google says that my Zimbra calendar is subscribed, but no events from my Zimbra calendar ever show up on my Google calendar.

I opened a Zimbra support ticket about this back in July 2023 and they gave me “internal enhancement number” ZRFE-1314, but they were no help and I can’t find that number anywhere here. I have several other subscribed calendars from other services that work perfectly in Google, so I don’t understand why Zimbra can’t.

I am attaching some screenshots showing my Zimbra calendar with events on it; the place in the settings where I shared the calendar; and my Google calendar with the Zimbra calendar theoretically there, but with no events showing. It surprises me that I’m the only person in history who has ever wanted to do this, but maybe there are others out there who find this as frustrating as I do. I’d love to figure out how to make this work.