Nextcloud Calendar - The Calendar app is a user interface for Nextcloud's CalDAV server.
Fantastical 2 - Fantastical, the Mac calendar app you'll enjoy using. Quickly create new events with natural language input and more.
Rainlendar - Rainlendar - Customizable desktop calendar
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
Etar - Material design open source calendar for Android.
Morgen.so - All-in-one Calendar, Tasks & Scheduler. Morgen is the single hub for everything that revolves around time management.