Are there any tenure limits? What is the 60-day clock?

Yes. Recruited or payrolled contingent resources working in Massachusetts and performing work that would be union-eligible if directly employed by Harvard have a 3-month tenure limit. (This may be extended up to 6 months if the contingent worker is backfilling an employee on leave who is expected back at work.)  The 60-day clock is applicable to people in this category and refers to this rule:  If this tenure limit is exhausted, there is a 60-day waiting period before the contingent worker may take a subsequent assignment in the same tub.  However, the worker may immediately take a contingent assignment in a different tub.

All other roles have an 18-month tenure limit, but they may be extended with the permission of local HR.  For these roles, there is no waiting period between Harvard assignments, even at the same tub.