Date Calender Starting Date of Field B should be equals to or advanced from the Ending Date of Field A
Try out Webform Validation module


  This module adds an extra tab to each webform node, allowing you to
  specify validation rules for your webform components.


OR in other case, write custom code, Use hook_form_alter() to apply custom validation

function mymodule_form_alter(&amp;$form, &amp;$form_state, $form_id)
{
    print $form_id;

    if($form_id=='webform_client_form_1') //Change webform id according to you webformid
    {
        $form['#validate'][]='mymodule_form_validate';
        return $form;
    }
}

function mymodule_form_validate($form,&amp;$form_state)
{
    $phoneval=$form_state['values']['submitted']['phone'];//where "phone" is field name of webform phone field
if($phoneval=='')
{
form_set_error('phone','Please fill the form field');
}
/** THEN USE REGULAR EXPreSSION TO VALIDATE IT  
In above example i have check if phonefield is empty or not
**/

}