Depending on each object setting, date fields can be one of the following:
•Time zone dependent—Converts dates entered by other users to your time zone. If you enter a date in a time zone dependent date field and a user from a different time zone views the date, the date will be converted to that user's time zone.
•Time zone independent—Dates entered by another user are not converted according to time zone, regardless of your time zone setting. This means that dates should show up "as is", regardless of a user's Time Zone.
Some System Date fields will also be set to be Time Zone Independent by default.