Default date field values like "today +10 hours 30 minutes"
The best way to control the default output of form elements is through the drupal Form API.

You have two scenarios:


If you created the form with the form api and have a date popup in your form, you can use the method described in this post to set the #default_format of the #default_value of the form element.
If you didn't create the date popup in the form element with the form api, meaning you added it as a field to an entity through site configuration, you can still make a small custom module solution.  Make a custom module and implement hook_form_alter, find the field in question and set the #default_value using the method mentioned in #1