Based on my initial theory of what the prosector is doing he is doing just fine.
His job is not to provide or argue with the bug guys dates. His job is to provide another estimate from a completely different field.
The strategy I assumed the prosecutor is doing is two-fold. Part one is to have another method of establising a date that doesn't rely on bugs.
Part two will be a bug guy that provides a more specific date based on bugs. Both will contradict the original bug guys.