Despite having my own e-mail server I still use Google Calendar for some things. I have an ICS file for the calendar for the Covid vaccination clinic I’m volunteering at. I ran into some frustrating sync problems when I tried to import it into my calendar. Google Calendar’s ICS sync process takes up to 12 hours, which was frustrating. I also had some mobile clients that wouldn’t even see the calendar imported from the ICS file.
I luckily found this post from Derek Antrican on stack exchange that outlines a script that you can configure to run at any given interval which will take all events in that ICS file and add/update/remove your calendar to match. It works beautifully. It’s a Google Apps script that you must copy into your own Google Scripts account to run.
First, go to the script here. Then go to Overview (i) and click “Make a Copy” in the top right (page icon.) Once the scripts are copied to your own script.google.com account, follow the instructions for configuring the script for your desired ICS URLs and other options, then click run.
My calendars are all synchronized and happy now.