Custom date range format on a node by node basis
You just need to add template_preprocess_node(&amp;$vars) to your template.php and check the value of checkbox that is controlling the date format

Date object store start date as value key and end date as value2 key. You need to check on that.

if($checkbox) //checked  
  //display one format
else
  // display another format


You can use php date api functions to format your date