Time Calculator

Divide the minutes by 60, not by 100, which is the mistake that quietly shorts invoices. 45 minutes is 45 ÷ 60 = 0.75, so 7h 45m is 7.75 hours; writing 7.45 undercounts by 18 minutes, and across a month of billing that's real money. The cheat sheet worth memorizing: 15m = 0.25, 30m = 0.5, 45m = 0.75, and every 6 minutes is exactly 0.1.

Because time is base-60 and everything else you add is base-10, so the carrying rules clash: 1h 45m + 2h 30m requires carrying at 60, giving 4h 15m, while your calculator, treating the numbers as decimals, happily reports 3.75 + wrong things. The adder here does base-60 carrying per row and keeps a running total, which is also why it shows raw minutes: multiplying a rate by minutes ÷ 60 avoids the whole problem.

Flip a row's plus to minus. The running total subtracts that row. The everyday case is unpaid breaks: 8:00 of shift, minus 0:30 lunch, minus 0:15 break = 7h 15m payable. Totals can legitimately go negative when comparing (planned versus actual, say), and the calculator shows that rather than clamping to zero, since a −1h 20m answer is information, not an error.

The result crossed midnight: 10:30 PM plus 3 hours is 1:30 AM the next day, and the calculator says so explicitly instead of just displaying 1:30 and letting you mis-plan. It matters constantly for overnight shifts, flight arrivals, and 'take the next dose in 8 hours' math. Subtracting works symmetrically. 2 hours before 1:00 AM shows 11:00 PM with −1 day.