You'd used "<= -1" to test for negative numbers, which doesn't make any sense if you're testing floating-point values (which could be -0.5 etc.) and why would you want a quadratic equation solver that only accepts integral coefficients? ("< 0" is more natural even when testing integers, by the way). So, figuring you didn't believe in the in-between values, I went ahead and rounded my start and end times to the last hour for billing purposes.