I have never owned a Clie, so I don't know if the Palm Desktop version that it uses has this ability. But in the Hotsync conduits on your computer, you could try to set Desktop Overwrites Handheld on the Datebook hotsync conduit.
Also in the Palm Desktop, there is a menu command somewhere that can check for damaged records, which also might help get the Datebook database back onto your handheld, if it contains a corruption. I have not used it in a long time, so I can't tell you the exact menu path.
Good luck!
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Former HP Expert (Palm/webOS)