Use it for questions about handling date or time through any module. Don't use it if you are already using "calendar" as tag.
In Drupal 7 and earlier, Drupal core didn't provide any entity/form fields to handle date or time, which are implemented from the Date module.
Starting from Drupal 8, the date/time field is now included in Drupal core, and implemented from the Datetime module, which also implement a view argument plugin via the DayDate class. Drupal core also includes a DrupalDateTime class.
