I would like to add my voice to the chorus saying how important custom calenders are (note the plural).
I run my own world and I use many different calendars. The old Empire had one calendar, the new one (thousands of years later) has another. The countries to the south use yet a third calendar and so forth.
This is wonderful when setting up plots. Like the old prophesy refering to some dates in an old calendar and the players have no idea if the things have happened long ago or are supposed to happen some time in the future. Figuring out how to "translate" the old calendar to the present calendar can be an entire plot in itself.
All this just to say, that I would really like the possibility to have multiple custom calendars in one realm and being able to write one date in one calendar and get the corresponding date in all the other calendars I have.
Furthermore being able to
easily choose which calendar will be used to show all the date information in the realm. So I can say "Kragôrn Imperial Calendar" and I can see all dates according to this calendar - then I can switch (by an easily drop-down menu?) to say "Late Raez Federation Calendar" and all dates will change to this calendar.
I assume that programwise this would require a generic calendar that all custom calendars are referring to, so it "just" becomes a matter of "simple" translation. I know, it is much more complex than I make it sound.
I would - like everybody else - like to get my cake now AND eat it now as well, but it is important that the calendar system (hopefully with support for multiple custom calendars) is robust, well thought out and easy to use. If that means we will have to wait a while, so be it.