Created attachment 160587 [details] The Calendar System feature back in KDE Version 4 With »KDE4« and »Plasma4« the Calendar System could be easily switched within the settings of the Clock Combined Calendar Widgets: Gregorian, Hebrew, Mohammedian, Indian, Chinese, Julian, Japanese, etc. For each of several Clock Combined Calendar Widgets, any Calendar System could be set, which could be used in the same Panel or on the same Virtual Desktop side by side. See screenshots. Since »Plasma« Version 5 the Configuration and Integration of the Module »Date and Time« into the »Plasma« Desktop Environment has changed, along with the »Plasma« Clock Combined Calendar Widgets. From »Plasma« Desktop Environment Version 5 on there is no option anymore to use any alternate Calender System at all, if not by Installing a Localized Edition of a Distribution, apparently and probably so in »OpenSuse« and »Knoppix«. Considering the settings with the KDE3 Fork Desktop Environment Trinity, the Calendar System was already switchable with Version 3 of KDE. Why was the old code abandoned? There are recently calls and efforts to rewrite this feature, notably for an integration of Arabic Calendars: https://bugs.kde.org/show_bug.cgi?id=429892 https://community.kde.org/KDE_Core/Astronomical_Calendars Calendar System Feature gone for instance in: Linux/KDE Plasma: OpenSuse 15.4 (available in About System) KDE Plasma Version: 5.24.4 KDE Frameworks Version: 5.90.0 Qt Version: 5.15.2 Calendar System Feature still exists for instance in: Linux/KDE Plasma: Kubuntu 14.04.6 (available in About System) KDElibs Version: 4.13.3 Qt Version: 4.8.6 And even more importantly: why was the old code forgotten?
Created attachment 160588 [details] But in KDE Version 5 the Calendar System cannot even be changed globally
Created attachment 160589 [details] KDE Version 3 forked into Trinity Desktop Environment: Fewer Calendar System Choices yet in System Settings
Please update to the latest Plasma (or upgrade to Leap 15.5). The alternate calendar was brought back in 5.26. Thanks
(In reply to Fushan Wen from comment #3) > Please update to the latest Plasma (or upgrade to Leap 15.5). The alternate > calendar was brought back in 5.26. Thanks Tested with: OpenSUSE 15.5 KDE Plasma 5.27.4 KDE Frameworks 5.103.0 In the newest Clock and Calendar Widgets there now indeed is an option named »Alternate Calendar Plugin«. Compared to the former Clock and Calendar Widgets in KDE4, the Calendar System cannot be switched seperately in every Widget anymore. With the new »Alternate Calendar Plugin« a second Calendar System can only be used beneath the same standard systemwide Calendar System in any Clock and Calendar Widget, and is displayed below the standard Date in a different color. Also, some of the alternate Calendars with the »Alternate Calendar Plugin« don't work with my installation. Only the Persian, Islamic and Julian Calendars work, the Hebrew, Indian, Chinese, Milankovic Calendars don't, no matter changing installed or activated locales. See Fotos in Attachments. The parallel Calendars from KDE4 provided specific views for special months and weeks, and the standard systemwide Calendar System could be switched, too.
Created attachment 161123 [details] The »Alternate Calendar Plugin« in Plasma 5.27.4 only provides choice for an extra Calendar beneath the standard
Created attachment 161124 [details] In KDE4 the system wide calendar system for file management etc. could be switched too
You're not going to get back exactly the old stuff; there was a reason why it was abandoned, as throwing it away without one would be nonsensical. I know it's not a satisfying answer, but it is what it is. This bug report can track the fact that the standalone Calendar widget doesn't have the Alternate Calendar functionality of the Digital Clock widget.