The Family Medical Leave Act was enacted to allow an eligible employee to take up to 12 weeks unpaid time in a 12 month period for the birth or adoption of a child, to care for a family member or for themselves for a serious health condition.  I can see how the rehab would qualify if the reason you went was associated with drugs or alcohol, which is a serious health condition.  But serving time in jail  - although a result of the condition - would most likely not qualify.  Be honest and up front with your employer and explain that you are taking all the necessary steps to getting clean.  That you made a mistake and that you are on the right path.  Ask to take a leave of absence without pay.  And ask them to hold your job - in writing.  Good luck.