Overtime is counted inside a work week — a fixed, recurring seven-day period your employer designates. It does not have to start on a Sunday and it does not have to match your pay period; it is printed on your stub, and it decides which hours land past the threshold. A week that looks like overtime on a calendar can stop being overtime once the real week boundary is applied.
The premium itself is arithmetic, not judgement. Every hour is paid once at your regular rate; overtime hours add half that rate again on top, which is what makes them time and a half. Payroll systems write it as two lines for the same reason this calculator does — the base pay and the premium come from different columns on the stub.