What is the custom date format to display Days or Months ago in date field?
I think it's best for you to use the Timeago module: https://www.drupal.org/project/timeago

First, it provides a timeago date formatter option for use with dates anywhere, which answers your need.

Second, it solves the caching and correctness of time-ago dates problem for you automatically. Even though you don't mention this in your question, this is important as it's best to cache pages in live production in one way or another.

Good luck.