Module development and code questions

i have read https://www.drupal.org/node/2941420 and i can upload a image file then use it in article.

but i can not use it as video field after i upload a video file. 

(my drupal version is 8.6.0-beta2)

I use the media type ( drupal 8 core module) for my video field , this is a Entity reference.

Hello,

I am at a loss, I am trying to change a form element by using a standard callback, but my form is rendered in a theme / template.

Now I don't know how to get the callback to work, as the form is rendered already.

Here is the standard call:

Sorry, if it is something very obvious; I'm not even sure, is it Drupal problem or just my stupidity...

I made flexslider slideshow and put that in the header background. (Example: http://liiga.ee -- it's not configured jet, but it basically works). I'm using 0point theme, I made new slider.php module, slider.js module and modified little template.php and page.tpl.php...

Do Views pages store a timestamp of when they were updated?

When you update a page of a view, ie make some changes and save, is there some timestamp stored somewhere in the views code?

This doesn't sound like a developer question, but I think it is important from the version management viewpoint.

Here I am again asking for help....
I succesffuly created a date via computed field - due date. Thanks for the help of this forum especially wombatbuddy.

Now I am again stuck to this: I want to create a calendar using the 'Due Date' in my computed field. But I cannot find the 'Due Date' field in the views template add calendar.

Is view calendar cannot recognized a date based on computed field? Is there a workaround on this? Or is there another altarnative? 

Is there a way to create an empty link in D8? What I want specifically is

<a href="javascript:void(0)">Link</a>

Url::fromUserInput('#', ['fragment' => '#']) creates an anchor tag with ## link.

Thanks