Added a Summary method to RegisterableDateTime.
Added a Summary method to RegisterableDateTime.

registripe/registripe-core,silverstripe-australia/silverstripe-eventmanagement,souldigital/silverstripe-eventmanagement,silverstripe-australia/silverstripe-eventmanagement,stephenmcm/silverstripe-eventmanagement,burnbright/silverstripe-eventmanagement,burnbright/silverstripe-eventmanagement,souldigital/silverstripe-eventmanagement,stephenmcm/silverstripe-eventmanagement