This is what I would do, instead vi(or what ever you use to make a text file) a file and call it something like "cron.uptime"
30 22 * * * /usr/bin/cron.uptime >> /var/log/uptime.txt
You dont need the & in there.
and then in command line type:
# crontab cron.update
and then it should run
The problem you might be having is that you did not make this crontab as root, and you wouldnt have permissions to write /var/log/
So either make this crontab as root, or make it write to somewhere else