I always remember to change my watch when I travel abroad or when we switch from BST to GMT (or vice-versa) but I nearly always forget to change the clock on the digital cameras. This usually doesn't cause too many problems other than annoying me when I realise I've forgotten just after taking a few hundred photos.
When I put together the highlights programme for the British Formula 1 race I was using photos from two separate cameras and so needed to ensure that the clocks were in sync. Unfortunately they were both out by an hour and about five minutes different from each other. So to put the photos in order I had to find a way of easily changing the timestamps.
After quite a long period of browsing the web I came across a really easy tool for doing almost anything with the EXIF information embedded in the JPEG files including altering the timestamps. The tool is unimaginatively called ExifTool. I actually used two commands, one to fix the timestamps and then one to rename the files based on the time they were taken. This allowed me to then put the photos from both cameras into the same folder and hey presto they were in chronological order and easy to import into the video editing software.