"moon is not right"? What is it s'posed to be?? Full, new? Please let us know !!
As HH says pigs are pretty accurate - so if the mate date was seen, the farrowing date will be within a couple of days of expected, regardless of the moon.
My view would be to humour them (don't fall out with the locals), and just keep quiet when you farrowing is not "right" by the moon