Actually the length of the Molad is off by about 6/10 of a second. And while that number is pretty good that error does add up. The time that the new moon should be seen in Jerusalem according to the molad now is closer to when it would be seen in Kabul.
The truth is that the Babylonians had very good numbers on the lunar cycle 2300 years ago, as did the Greeks (who got them from the Babylonians).
I actually recorded a podcast about the Hebrew calendar with my Rabbi for 365 days of Astronomy, it should air at some point in the next few months I hope.
For the record I have a BA in physics (including astronomy)