Add class for a date field
this might be what are looking for Adding CSS Classes to Fields in Drupal


  Implementing any type of modular CSS requires fine grain control over
  classes in your HTML. In my last post, we looked at how we can easily
  add classes to blocks in Drupal using preprocess functions. Now let's
  apply the same technique to fields. Fields are a fundamental building
  block of any Drupal 7 site. Fields contain content and content is
  king. Before we dig in and start classing up our fields, let's look at
  how they are rendered to the screen.