Ensuring accurate date and times on your photos
The most important and also most problematic aspect of geotagging [wikipedia.org] photos is ensuring the photos have an accurate time stamp. There are several helpful tricks to making this easy and you can always adjust the time stamp on your photos later if necessary.
Pick a time zone and stick with it
In general digital cameras do not record any time zone information to the photos and therefore there is no way to determine the true time on a photo. This ambiguity can become even more problematic when traveling between different time zones or adjusting for daylight saving time. For this reason is is best to simply pick a fixed time offset and stick with that, regardless of changes in daylight saving time or travel to other regions. For example, someone living in California may choose to always have their camera set to -08 GMT, even during the summer when daylight saving time is observed.
Many people find it most convenient and least ambiguous to set the camera's time to UTC (essentially GMT). Because your photos are geotagged, you can always determine the local time the photo was taken using the longitude and latitude. You can visit the National Institute of Standards and Technology to compare the time on your camera with the UTC offset given for different time zones.
Accuracy down to the second
Although the clocks in digital cameras are prone to drift and are often impossible to set to the nearest second, it is still possible to time stamp your photos with precision accuracy. Because GPS receivers have sub-second accuracy, you can take a digital photo of the time displayed on the GPS receiver and then compare the photo's time stamp to determine the offset. Typically one might do this after every major trip and then batch shift the times on all of those photos.