Check to be sure you have all your updates. What can happen as well, in the US, within the last couple of years they changed the days that daylight savings time takes effect. Could be that your software needs a patch to tell it that if the same is true for you. Al;so, in your clock, check your time zone setting. I've seen it happen that a system was older, needed that update I mentioned, and it was set to update time with the web, so what I think actually happened was that I had to check the time zone and turn off the updating so that it would stop what you are describing. One of those weird issues.
But to check the settings for syncing the internet time, click the clock in the start menu, click change date and time settings. Click on Internet Time, click change settings. In there there should be a box to uncheck to disable it syncing with the web. But check the time zone first. If it keeps time on leopard, the cmos battery should be fine, it would have to be a software issue.
And incidentally, at least on the 20 inch iMacs I've worked on, if the cmos battery dies, they would not boot. Just give you a series of beeps. Replaced the cmos battery, powered right up. Strange to me too......But that was actually what apple told me to check and it worked...go figure.