Migration time created of node
Most likely your source plugin isn't supplying the created field to do what is done in the reference article:


source:
  plugin: custom_article
destination:
  plugin: entity:node
  bundle: article
process:
  ...
  created: created


This part of the source plugin config is saying, "import article nodes by setting its created field from the source column created generated by the custom_article plugin". 

Adding: 

created:
  plugin: default_value
  default_value: 1389312000


Is telling the migration to "set created to 1389312000 as a default (if there's no source plugin data"

If you're copying the reference article's modules/migrate_custom/src/Plugin/migrate/source/Article.php, you'll need to make sure your plugin's query() method is correctly querying its source so that's available for processing and 

process:
  ...
  created: created


Is set in your migration config so that it's sourced correctly.