Modify Period End Date changes the dates for each daily time entry in addition to the period end date. The distribution of the time is always the same number of days from the new period end date as they were the old.  Modify Period End Date can only be used on unposted checks in the new.prt which have not been printed.

You can use Change Unposted Time to change the dates of the daily time entry appropriately after you modify the period end date of a check.

See How do I change the period end date on my Payroll checks? for steps to modify the period end date in Payroll.