I don't know, but they must have some sort of way to figure the timing. The only question is how accurate can they get?
If they can't get it down to the day, then the value of the test in a date rape case would seem to be small.
Sorry - I was being ironic. I know they have statistics on rate of hair growth, but I agree with your point that a test performed a month later would be unlikely to identify time of ingestion of a drug with the precision and accuracy needed in this case.
Therefore, a positive test would be very damaging because the general public would not understand that it means she had ingested the drug sometime/anytime and would jump to the conclusion that she had been given it within the very brief window in which she was in association with the lax team.
A negative test is very significant because it means that she had NEVER taken or been given the drug by the lax team, herself, or anyone else.