To be fair, the Drupal date module isn't just "the simplest helper module."
Off the top of my head, those 14k lines of code also include:
- a system-wide API for other modules to reuse - the most important part
- a few administrative UI variants
- hooks into several common extensions including
- ... and I'm probably forgetting several others
- tests for each
- Markups and styles for front-end
- hooks to override any of the above
- administrative options to override most of the above without a single line of code
It's not quite the same thing as a WP plugin that lets you add Date fields to posts.