Expense date is date selected by the user in the Date field on the top right corner of the Expense Sheet. It is used to identify the expense sheet for reporting purposes. The user can edit the expense date before submitting the expense-sheet and there is no relation between this date and the incurred date or time-sheet period.
Incurred date is the exact date when the expense was incurred. This can be selected in the Date Incurred field on the expense sheet.