The various timestamp properties (Date created and Date modified in the Standard Properties category, and Date digitized and Date taken in the Picture Properties category) let you enter what's known as a time shift string, which lets you perform relative changes to the existing values of these fields.
The format of the time shift string is either:
<date-shift> <time-shift>
when you want to adjust both date and time, or when you only want to modify the time:
<time-shift>
When one string is given it's assumed to be the <time-shift> string, which means you can't adjust just the date without specifying a time value as well (although the time value given can be 0 which means the time won't actually change).
The <date-shift> portion has three allowable formats:
The <time-shift> portion has three allowable formats as well:
What this means is that you can add or subtract years, months, days, hours, minutes and seconds to or from the current date value. For example, say you had just taken 100 digital photos, when you suddenly realize you'd forgotten to adjust your camera's clock for the beginning of daylight savings. You could fix this by specifying +1 (or just 1, since + is the default) for the time shift string. By the above rules,
This would add one hour to the current timestamp of the selected images.
Here are a few other examples: