Using LD_PRELOAD to make date return a specific date

I recently discovered LD_PRELOAD, a cool environment variable that
lets a library "intercept" system calls. For example, setting
LD_PRELOAD to /usr/lib/libtsocks.so lets tsocks intercept socket
connections and redirect them to a SOCKS proxy.

My question: how can I write a library that intercepts the
gettimeofday() system call (or time() or whatever the 'date' command
uses) and gets 'date' to return, say, "Thu Jan 1 00:00:00 UTC 1970"?

I realize this involves a couple of steps (writing a C "library" for
one), so any pointers are appreciated. My real intentions are more
complex (and sinister <G>).

-- 
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