March 1, 20,807 BCE. Searches farther forward, up to about 300,000 CE, reveal transits at roughly 50,000 to 70,000 year intervals. I haven't looked farther backward.

Stellarium, as released, doesn't work too smoothly with JDs less than zero; the JD-to-calendar conversion seems to assume that float-to-integer conversion always rounds down, when in C++ such conversion rounds toward zero. I plan to submit a patch Real Soon Now.