Because for some bizarre reason, the server host can't manage to keep the time set correctly. I won't embarrass them by naming them here but I don't think we'll be using them again!
Dyls
I think we may have the same host, our IPs are certainly very close, it may even be the same server (or they have more than one faulty motherboard which is worrying).
Anyway, I installed ntpdate (apt-get install ntpdate for Debian variants) and created the following script:
/etc/init.d/ntpdate
#!/bin/bash
echo "1" > /proc/sys/xen/independent_wallclock
/usr/sbin/ntpdate 0.uk.pool.ntp.org
and created the appropriate symlinks in /etc/rc*.d/ so this runs at startup, and it seems to do the job. The clock seems to stay right whilst the server is up, and when it goes down and backup, this script quickly sets the right date/time.
The underlying problem is the motherboard is f****d and they can't set the date or time, so each time it reboots, the VPSs inherit the wrong date and time. Unfortunately, after months of uptime, recently the servers on this host seem to be going down more and more.