The Earth travels around the Sun in our orbit at roughtly 1,112 miles per minute, or 66,705 miles per hour, 1,600,921 miles per day and roughly 584,336,234 miles traveled in our orbit in a year. Therefore if their calculations are off by
less than 2.79 minutes in the wrong direction we have a hit, and that's calculated for 35 years from now. That's an error in their calculations of only
.0000152%.
And remember, these are the same guys who forgot to convert from meters to yards on the Mars mission.
Oopsy!