Is this the solution ?
#28700
Please test.

I tested your solution, no change
I think this problem should be in the file
media / system / js / fields / calendar.js
Lines 1014 to 1042 that change the value "data-alt-value" when the calendar opens

This file has not been touched for years and is a dynarch project. I would not touch it even with twisters...

@rasouldanesh90 3.9.12 is a bit old, current version is 3.9.16, and 3.9.17 will be released soon. You should keep your Joomla always up to date because of security patches. Beside this, could you check if the corrections from pull requests #28596 and #17859 solve that issue? You should apply both changes together.

Also, please specify some dates concerned (in gregorian format)

This error appeared when you want to declare personal function to control date in persian
and we need return true date but returned null
In gregorian is true
But joomla team forget to return true data for those function

@balvard
It is impossible to do anything if you do not explain exactly (with code) what you want to do and where precisely, in your opinion, is the error in the js files.
fa-IR is using a custom calendar.js file which is installed in media/fa-IR/js/
Here is its uncompressed version
calendar-uncompressed.js.zip

Closing due to lack of response. You can comment here and we can re-open the issue.