add field with date field
Use Field Collections and add two fields to field collection and enable field collection field on your content type... Or Create two fields Flag and Flag date...


Flag
Flag Date


OR 

You can use revisions to track these changes in Drupal. When ever you change above field make content as a new revision so that you can track dates too...

OR 

Create a few more date fields, each for possible values to track the dates... use form api #states property to hide display fields based on selection of flag value..


Flag
Draft Date
Sent Date
Approved Date
Rejected Date