(Android) Calendars.
fossify calendar is somewhat great, but it doesn't have support for tasks (except in its own calendars that can't be synced)
And don't get me started on CalDav and trying to selfhost it. Baikal, Radicale.
Actually I think CalDav is the source of all of our problems. At least everyone I know won't touch it with a 2-meter pole.
And the reason why proprietary Calendars are so much better.
All I have is 2 Rooms full of Philips Hue Lights and A few Cameras.
I am always so envious of people that were already able to automate their Smarthome.
Next I want to automate the outdoor lights as well as the Garagedoor. But I think I'll wait for Thread because it seems more resillient than Zigbee.