The 40-hour week
Under the federal Fair Labor Standards Act, non-exempt employees are owed 1.5 times their regular rate for hours worked beyond 40 in a workweek. The workweek is a fixed, repeating 168-hour period, not a calendar week that changes with the schedule, so the 40-hour test always applies to the same seven-day block.
Overtime is calculated on the regular rate, which includes non-discretionary bonuses and shift differentials, not just the base hourly rate. A week with a production bonus therefore has a higher overtime rate than a week without one; the calculator prices the whole week at the single rate you enter before splitting out the overtime premium.