I've been building PayTimeHub , a set of free payroll and pay calculators - paycheck withholding, raise calculator, hours-to-decimal, time and a half. Money math looks trivial until you actually ship it, and one bug almost got through that's worth writing up. The setup Every calculator on the site takes a dollar amount, does some arithmetic on it, and shows the result back to the user. A raise calculator multiplies a salary by a percentage. An overtime calculator multiplies an h...