Actually, I think the use of the 17th is correct, because it is the last time it can be verified that Heather was seen. I don't know of course what information LE has but I'm sure they have interviewed the staff at the restaurant and anyone the date thought might be able to act as a witness after leaving the restaurant. But, after leaving the restaurant, there may be no other witnesses, or cctv, or any other independent way to verify Heather's whereabouts. All they have is the date's word. LE generally (that's generally) will use the last verified time. In this case, that's somewhere around the time the two left the restaurant on the 17th.
Hope that makes sense,
Salem