Some time ago a QA buddy asked whether the Linux uptime can be changed on the fly in order to study the behaviour of a closed-source application that would only exhibit a certain bug after two weeks of running.
I looked at sys_gettimeofday (Linux kernel 2.4) and learned that it depends on jiffies. So I wrote a small module that adjusts jiffies at will.
The technical article is here: http://spamchk.no-ip.org/kernuptime.html. The source code is here.