Drupal 8.x

I'm currently developing my first Drupal 8 module which fetch news messages from an old Drupal 6 site and show them (listing and detail) on my D8 site. It's nothing complicated and everything works fine, the only problem I have is the caching for anonymous users, so if I go to the page as guest it shows me always an old version of the page from the cache and clearing the whole cache is the only way to update the pages. How can I manage that for example the cache is just valid for twelve hours? If I set the caching maximum age under performance to this time that doesn't change anything.

Hello,

I have an issue with block beign cached for anonymous users. It does not happen on my site that still uses Drupal 8.3.7, but on the other site with Drupal 8.4.4 it does.

The problem is that I have a countdown block written by me in a custom module, it counts down time from current timestamp to the chosen date.
The code for rendering the block looks like this:
 

Dear Drupal family,

I tried to do as much training videos for Drupal 8 as possible. I looked at modules like webform. But I am still a bit lost. Can you help me and recommend modules or another basic approch to be able to build the website I will describe in the following text?

I installed default content module for import data in json format. I have my json here. [my_module/content/[entity_type]/anything.json]

After Profile installation completed, I enabled my custom default content module with content sample json for node entity.

I am not getting any error, but content not imported. So, I would like to know, how should I import my default content using  Default Content for D8 in brief.

Dear community,

I need to put a drop down of taxonomies in a Header of a View. When I select that taxonomy it takes me to a URL that I tell you. I need this because I have a "View" with a "contextual filter" for taxonomies and I need to select the "drop down" take me to the "URL" that I specify in the header to go browsing the content that they have those taxonomies. I do not know if there is any module capable of creating that "drop down" of taxonomies to point to a "url" or how it could be done by programming.