no problem.
FirstDayOfPeriod function to the rescue. It was really designed for your exact use case, like a very typical 2 week pay cycle than never changes come hell or high water.
Change the Key field of your pay periods table to be the field [Beginning Date]
The on the details time entries make a formula field for Pay Period beginning date
https://login.quickbase.com/db/6ewwzuuj?a=dr&r=bm&rl=w5jFirstDayOfPeriod([date field], Weeks(2), Date(2018,12,31))
Use that field as the reference field on the left side if the relationship were 1 pay period record has many time entries and then make your summary field.