Dealers will try to convince people at trade time of all sorts of things. It is a negotiation tactic. If you are a good negotiator and the condition of your car is decent I seriously doubt it will make any difference on your trade in if a car is built Dec 04 but is complied Jan 05 (as opposed to built Jan 05/complied Jan 05).
Don't accept everything dealers say to you - eg oh this is the 2004 model, even when it is the 2005 series etc.
To suggest dealers go by calendar year to assess market value doesn't make sense either. In that calendar year there could be 2 different series of the same vehicle. Sometimes those upgrades are significant (eg the 06 WRX - totally different front end).
If I buy the 2005 WRX (built in 2005) and also the 2006 series WRX (built and delivered in 2005) - will they be worth the same as both built in 2005? Of course not - the series of the vehicle makes a difference.
It all comes down to how you negotiate - makes much more difference than any red book or so called book value of the car according to the dealer.
