WASHINGTON — The day just got a tiny bit shorter because of Friday's giant earthquake off the coast of Japan. NASA geophysicist Richard Gross calculated that Earth's rotation sped up by 1.8 microseconds. That's because of the shift in Earth's mass caused by the 8.9-magnitude earthquake. A microsecond is one-millionth of a second. Friday's change in rotation speed is slightly more than the one caused by last year's larger Chile earthquake. But 2004's bigger Sumatra earthquake caused a 6.8-microsecond shortening of the day.