On the down syndrome screening test my due date was miscalculated. does it matter?

Not the issue: Studies show that most people don';t change their minds if test positive, so no real reason to do test unless your OB asks you.

Several test types: There are several types of testing availble, from sonogram, chromosomal testing from chorionic villus or amniocentesis and new blood tests. A small difference in fetal age will not likely invalidate all of the tests.