Customizing Calendar View

[limitless]

Hello,
How can i change the date display in the calendar to include another type of date - Please see screenshot for example
The new date is calculated in PHP with the following code :
$jd = gregoriantojd(10, 8, 2002);
$newdate = jdtojewish($jd, true), PHP_EOL;

Can someone help me understand where this code goes and how to display + make it in an update safe manner
Thank you

[Andorxor]

You could add the file custom\Espo\Custom\Resources\metadata\clientDefs\C alendar.json with the content

{
   "calendarView"."YourCustomView"
} 

[limitless]

Thank you Andorxor - i will give it a try - is there any documents anywhere to learn on how to work with this or an example i can follow somewhere?
Much appreciated!