I was surprised to find that after the clocks changed on 26th October 2008, all of the dates on my billing records were showing up a day early. Invoices paid on 28th September showed as being paid on 27th. I tried it out on 3 different servers as well as my local PC. On the 3 Linux servers, it always calculated the wrong date, but on my PC (running Windows XP), it showed the correct date. The explanation was fairly simple, but I found it infuriatingly difficult to fix!