You need an accurate timing resolution of less than 1ms for useful groove nudging, and it's really tough to get PCs or Macs to be that precise on any OS.

If you use a high level language like Ruby, js, or Python the actual timing will be randomised anyway.

For maximum accuracy you need a low level scheduler written in C++ with a few special tricks. Of you can farm out the timing to something like JUCE - which isn't quite perfect, but is good enough for most applications.