A whole second every 300 million years? So about 3.33 (repeating of course) nanoseconds a year. Seems like a lot, is this the most accurate created so far?
We can measure time more accurately than anything else, but it's still not good enough for some purposes you could imagine. You can't, for example, use two independent clocks to keep two gigabit transmitters in phase without feedback for more than months.
Did you skip this sentence: According to BIPM data, NIST-F2 is now the world's most accurate time standard.