How to modify the calendar type?

I want to have date displayed in both Hijri and Gregorian calendar systems. How can it be done? Since I’m quite new, I’m curious to know if these codes (e.g. the Analog Clock) are stored somewhere as plain-text or they are binaries?

It might help if you tell us what desktop you are using, since different desktops handle this differently.

I’m using the default desktop, which I believe is Plasma 5.