An employee who is sick or is injured may apply for a leave without pay when the employee's sick or injury pay has been exhausted and extended sick or injury pay has not been granted. The request must be accompanied by a statement from the attending physician verifying the necessity of such leave. The leave may be granted for periods of thirty (30) calendar days or more, depending on the condition of the employee, not to exceed two (2) years from the date the employee's sick or injury pay has been exhausted, unless this period is extended in writing.