The best open source alternative to Google Calendar is Etar, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 100 alternatives to Google Calendar and many of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Google Calendar are Calendar Lock PEA (Free, Open Source), Lightning Calendar (Free, Open Source), Simple Calendar (Freemium, Open Source) and Nextcloud Calendar (Free, Open Source).
Why? I wanted a simple, material design, state of the art open source calendar that anyone can improve on. Features: - Month view. - Week, day & agenda view. - Uses Android calendar sync. Works with Google Calendar, Exchange, Etesync etc.
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Etar vs Google Calendar opinions
Almost everyone thinks Etar is a great alternative to Google Calendar.
Password Encrypted Calendar: Calendar Lock PEA - Displays iCalendars that are plaintext only in RAM, but otherwise always encrypted on the device. - Uses autheticated encryption and a memory-hard key derivation function for the password.
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Almost everyone thinks Calendar Lock PEA is a great alternative to Google Calendar.
Organize your schedule and life’s important events in a calendar that’s fully integrated with your Thunderbird or Seamonkey email. Manage multiple calendars, create your daily to do list, invite friends to events, and subscribe to public calendars.
Most users think Lightning Calendar is a great alternative to Google Calendar.
Kontact is the integrated Personal Information Manager of KDE. Kontact suite unites mature and proven KDE PIM applications under one roof. Thanks to the powerful KPart technology, existing applications are seamlessly integrated into one.
GNOME Calendar is a simple and beautiful calendar application designed to perfectly fit the GNOME desktop. By reusing the components which the GNOME desktop is build on, Calendar nicely integrates with the GNOME ecosystem.