To make sure your machine’s time is up to date all the time, install ntp (apt-get install ntp, or do it how your machine does it), and then add to /etc/ntp.conf:
server uk.pool.ntp.org
server uk.pool.ntp.org
server uk.pool.ntp.org
Yes, that’s the same line three times. See the usage instructions for why. Thanks to Peter Oliver for explaining this to me.