This article explains how to export .ics file from KOrganizer into GNOME Calendar in Ubuntu. It also introduce the integration between the GNOME Calendar into Ubuntu time & date in the top panel. This article makes use of Ubuntu 16.10 Yakkety Yak and GNOME Calendar 3.22.1 from GNOME3 Staging PPA.
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Exporting from KOrganizer
In KOrganizer, open menu File > Export > iCalendar > type the file name as you wish > OK. The file name should be entailed with a .ics extension.
Importing to GNOME Calendar
To import a .ics file, open GNOME Calendar > open menu Calendar > a Calendar Settings window appears > click Add > click From File > choose the correct .ics file you want > OK.
Known limitation: I found that custom colors and categories from my KOrganizer settings can not be recognized by GNOME Calendar. I can only choose one color for the whole calendar imported.
See The Integration
All schedules and events inside GNOME Calendar are integrated with the desktop Clock Applet on the top panel. This way, you can always check your schedules at anytime without opening the Calendar program.
About iCalendar
iCalendar is a global standard created by Internet Engineering Task Force (IETF) to allow internet users exchange tasks, schedules, calendar, or meeting requests, through email or a file format called .ics. Software used in .ics editing/creating is usually organizer or email client, such as KOrganizer, GNOME Calendar, or even Google Calendar. Read more about iCalendar here https://en.wikipedia.org/wiki/ICalendar.