Chris,
Look at 1904.7(b)(3)(i) and (ii). Based upon those paragraphs, I would use the 5/01/2008. Also, take a look at the OSHA Form 300, Column D states "Date of Injury or Onset of Illness". Columns K and L describe the number of days on restricted duty and number of days away from work. Conceivably, you could have an employee injured on 5/1 goes on restricted duty 5/7 and is out on 5/14. In this case, column D would be 5/1. Column K would be from 5/07 to 5/13 and column L would be from 5/14 to the present date. I would keep copious notes on this situation. From my experience, it sounds like you have the start of a worker's comp lawsuit.
MikeCJ