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Calendar and tasks sync between devices
(lemmy.ml)
F-Droid is an installable catalogue of FOSS (Free and Open Source Software) applications for the Android platform. The client makes it easy to browse, install, and keep track of updates on your device.
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Davx⁵ has Mac OS caldav carddav server integration so you can sync with it.
https://www.davx5.com/tested-with/calendar-and-contacts-server
On android you can use simple calendar or etar to view calendars and "tasks" or jtxboard for tasks/journals