Using Date Hired as the original Start Date and Adjusted Hire Date as the rehire date of effective hire date, decremented by any leaves of absence?

We monitor the Adjusted Hire Date in order to see if anyone was classified as a rehire and thus treated as a new employee. The scenario you portray is the exact way in which our solution works: 

When adjusting the Adjusted Hire Date field to a new rehire date, the employee will be put into a new initial measurement period for testing. This has the effect of removing the person from any standard measurement period testing and restarts their eligibility determination.