Yes, Thats correct. Your Tier Score will reset on 1/1 of every year, If you gain 10,000 Tier Credits let say in 2007, you have diamond from the point you got 10,000 tier credits and the rest of 2008. In march/April of every year, they do whats called "Tier Status Renewal". Bascially renewing you for the next year, providing of coruse you earned enough tier credits in the previous calendar year to preserve your status.
To Answer the question about reward credits going faster, Yes, You will earn reward credits alot faster, mostly because of bonus reward credits, they will have promotions throughout the year where you swipe your card and you can win instant cash & reward credits. However they will NEVER have a promtion for you to lets say earn "1000 Bonus Tier Credits" Tier Status, Gold/Platinum/Diamond/Seven Stars, is desinged to show the casino how "IMportant" you are to them.
I know alot of people here, dont like Harrah's I persoanlly love their rewards program. I have gotten more hotel offers then I know what to do it, and many other awesome offers, the best is the diamond lounge, GREAT, FREE, Food and Drinks Available to you 24/7. And the Casino I play at has the bext BLACKJACK Game within 100 miles of my house.