It should be easy.
Can just replace calendar to datetime field in all XMLs, or extend CalendarField from DatetimeField field class.

However, 2 questions opened:

• We need to agree to drop support for non-gregorian calendars. And (in theory) we can provide API for translators to load their JavaScript to alter datetime input to load non-gregorian calendar.
• The translations is impossible. Joomla provides multi-language interface, however the translations for the native date input will be from the browser. And for example when you try to use Joomla on PC of your friend from France and login to Joomla selecting English interface the Date input will be French. For me it is not a problem, but this can make unhappy some peoples.

I actually see it as a bonus that the language of the calendar will always be in my language - what I am not sure of yet is if the ui supports the correct start of the week for that local.

what I am not sure of yet is if the ui supports the correct start of the week for that local.

Should work. And depend from User OS configuration.

I am not a representative of the groups that use the Solar Hijri calendar.
I just wanted to mention regarding
#46442
that there is also this link:
https://babakhani.github.io/PersianWebToolkit/doc/persian-date/

But I don't know what is correct. I only know that fundamental changes and adaptation put a lot of pressure on extension developers.
Therefore, perhaps a gradual change with simultaneous support for the old calendar could be a solution, if it's feasible.

The Joomla Maintainer Team decided at the 27.05.2026 meeting to accept this feature.
Minimum Target Version: 7.0

We need comprehensive testing across a diverse range of calendar types to validate that it fully replaces our current calendar system and addresses all necessary use cases.

This one also exists that's lightweight,
but it seems this one also has limitations regarding date range selection within a single calendar:

• Demo: https://majidh1.github.io/JalaliDatePicker/
• Repository: https://github.com/majidh1/JalaliDatePicker

My idea would be as follows:
By default use native datetime input, and we provide an API for translators to load their JavaScript to alter datetime input to load non-gregorian calendar. Just as a fancy if/else in the layout or something.

This way we delegate part of work to people who know their calendar.
That also means Jalali script/assets will come together with translation, and we need to help them to make it work.