uninitialized constant Date ?

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I've just set up a new Ubuntu 8.0.4 system and installed Ruby 1.8.6.  I'm getting 'uninitialized constant Date' errors in irb when I try to do anything with the Date library.  Time works fine.  I've never had a problem like this and would appreciate any advice on how I might go about debugging / fixing it.

TIA,
Bill
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