Please note that non-60Hz displays are surprisingly common (not just "gamer monitors" with 120, 240Hz etc..., but also odd refresh rates like 72, 75, 85, 90 and 100Hz)

Ideally the 'render clock' would query the display refresh rate from the operating system, but this is harder than it sounds. The next best option is to measure the frame time, and then round to the next "common" refresh rate (AFAIK that's the only way it works on the web for instance).