The reimbursement request had one or more send-backs. To which submission date does the policy apply?

The initial submission date. 

Expense Reports are submitted in a two-step process: Expense Report and Personal Time Questionnaire. The "initial submission date" is based on the personal time questionnaire's initial submission date. 

All other dates from multiple send-backs and resubmissions, do not override the initial submission date, as Workday records every submission (Oldest - Top of Page, Newest - Bottom of Page).