Drupal 7 - Webform - Validation

I am using Drupal 7 installation and am using a webform to manage a submission of some data.

The fields are as follows:

1. Date field which captures a date input
2. Two textfields which capture numeric values
3. Checkbox which captures a customer acceptance

I want to be able to allow the form submission based on whether:

• (1) is greater than a certain date
• The sum of values in (2) are greater than a certain value
• Checkbox in (3) is checked

What would be the cleanest way of accomplishing this in Drupal 7?

I ended up accomplishing this using hook_form_alter() and a callback function wired to $form['submit']['#validate'][].

function mymodule_form_alter(&$form, &$form_state, $form_id) {
  if ($form_id == 'myform') {
    $form['submit']['#validate'][] = 'validation_function';
  }
}

function validation_function($form, &$form_state) {
  // Validation logic here
  // If in validation failed set error message here
}