as discussed by @calamarimhere or by @liborbhere, the birthdays of contacts are not available in offline calendar.
So that means you MUST sync your phone to have the birthday in your calendar. But some people (like me) doesn’t want to sync phone with cloud for many valid reasons (security and so privacy, …).
So create himself event for the birthday of your contact in offline calendar is quite time consuming and even more when you need to backup the calendar (what you can’t do without cloud). This means you have to recreate birthday calendar at each time it has been wiped by reinstallation or data cleaning for any reason!
So the idea is quite simple (to propose): allow calendar for contact’s birthday in offline calendar!
If you like the idea, support it using like or add comments.
You could just use another calendar app. Simple Calendar (from F-Droid) does the job very well, has several widgets and adds another nice feature: local holidays.
Thanks guys for your inputs.
So I tested Simple Calendar and so I think that for the moment I will switch to this app.
But I hope Calendar will be improve in the future to avoid to have to install other apps.
Yes, Birthday Adapter is the one I used in the past.
There was an issue regarding years or something. A build with fixes was posted in this thread, which I used. That was back in the Oreo days though.
Indeed Birthday calendar works fine (with the small bug evoked by @marcdw).
In that case, instead to reinvent the wheel, why not implement this app inside Calendar??