Google Calendar has a Week Number calendar that neatly adds the week numbers as little icons to my calendar. However, I want the numbers to start halfway through the year, not from January.
Is it possible to change which week is week 1 OR is it possible to make my own calendar with those little icons?
I don't think its possible natively. Here's a Chrome extension for this.
This extension allows you to set a date which the first week starts on. The week number will then reflect this. Check its description in the web store for more details.
Works in Chrome/Edge (just install from the Chrome Web Store)
