Here is the long version:
When the Coinage Act of 1965 was passed, it became mandatory that the Mint continue to use the 1964 date on all 90 percent silver coins (half-dollar coins, quarter-dollar coins, and 10-cent coins). Therefore, all of the 90 percent silver coins that the Mint manufactured in 1964, 1965, and 1966 bears the date 1964. The last of the 90 percent silver quarter-dollar coins was struck in January 1966, the last of these 10-cent coins was struck in February 1966, and the last of these half-dollar coins was struck in April 1966. The Coinage Act of 1965 also made it mandatory that the clad coins be dated not earlier than 1965. Therefore, all of the clad coins actually manufactured in 1965 bear the 1965 date. All of the clad coins made through July 31, 1966, bear the 1965 date. The first clad 10-cent coin was struck in December 1965, the first clad quarter-dollar coin was struck in August 1965 and the first clad half-dollar coin was struck in December 1965. In December 1965, the decision was made to change the 1964 date on the five-cent coins and the one-cent coins to 1965, as one step in catching up on normal coin dating. From December 1965 through July 31, 1966, all one-cent coins and five-cent coins were struck with the 1965 date.
Quarters and dimes are composed of cupro-nickel clad, with a pure copper core, and an outer layer of a 75 percent copper, 25 percent nickel alloy.
Nickels are made from the same 75-25 alloy, and the cent, once a copper coin, is now composed of copper plated zinc. These cents are less expensive for the Mint to manufacture, and at 2.5 grams each, weigh about 20% less than the cent previously minted of 95 percent copper and 5 percent zinc, which weighed 3.11 grams.
SCOURCE:
http://www.usmint.gov/faqs/circulating_coins/index.cfm
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