Hello everyone,
Before 2 weeks I have posted a suggestion at ERPNext Github to make the Date Field be based on and restricted to the Default Fiscal Year since the Posting on date field becomes problematic especially when entering historical data.
Then I thought of solving this problem myself, so I started by creating a Client Script for specific forms, but today I thought of creating an App/Extension that modifies the Date Field handler and I did.
To share it with everyone, this is what I have done.
Note
I didn’t have the chance to test it myself, so if there is anyone who would like to give it a try, I will be very grateful 
Best regards.