I have a giant Rincon men's mountain bike (serial number GU675673) with a straight crossbar (not inclined) like most I've seen online. I am trying to determine the date of manufacture in order to determine its value today.
Unlike most automobiles bike model years are best described as "fluid". Many years the only difference was what colors were available. As the model year comes to a close, parts may become unavailable so parts slated for the next model year may have been used. There are several resources I use but mostly rely on Bikepedia and BicycleBlueBook. Both of these require an estimate of the age to get a close approximation. You may still be no closer than a 2-3 year range. In general used bike prices will vary a considerable amount based on region and time of year. When most bikes get to be 10 or 15 years old there is little value added for a COMP versus a SPORT model. A check of your local Craigs List may give you an idea of local value. You have to realize that the listings will have an asking price which may be unrealistic.