In the Southern, article 49 (local supplement) page 208 section 1.
any part-time employee in the case of emergency (and the Managements lack of planning IS an Emergency!) work beyond the 5th hour, the employee shall be paid time and one half for all hours for all hours worked past the 5th hour per shift"
True, start time can only be adjusted by 1 hour, (start time at 6, you can only be changed to a start time of 5) But, suppose they want to start the sort at 2, I feel that might come under the "Extra work" provision of the contract. So, they can ask for volunteers for the earlier start time, and process what they can with the reduced workforce.
The Co. can at the start of the sort indicate the day is projected for 6.5 hrs.
The employees are all expected to stay until the work is done.