The events of the animated series are said to take place during the final two years of the Enterprise's five-year mission. Source: Internet Movie Database.

I own the series on DVD. As I watched the episodes multiple times, I made some insights:

• All the characters vocal inflections are toned down (Shatner's over the top delivery of lines is gone.)
• The cartoon wasn't totally dumbed down for kids. Dialogue included use of some big words at times and complex technobabble. This is probably because later on adults enjoyed the show too.
• Like He-Man and The Masters of The Universe, the show used an animation process (not sure of the field terminology) in which they retraced the cells so that different characters can do the exact same poses. This is most notable in the running scenes. They all run exactly the same.

• An entirely new soundtrack. None of the incidental music from the live series was used.

  They did a pretty good job within the budget that they had.