Why are modern nickels minted with an 1803 date?

The 1803 is not the minting date, which is traditionally marked on the obverse, or face, of a coin. The 1803 is there because the reverse is a copy of the 1803 Indian Peace Medal which Lewis and Clark presented to Native American tribe leaders as they explored the Louisiana Purchase.

This particular design was only minted for the first 6 months of 2004, as part of the Lewis and Clark bicentennial. The 2004 date is on the other side of the coin; this was the last year that the nickel carried the original left-facing portrait of Thomas Jefferson which first appeared in 1938.