vBulletin Manual - Date and Time Options

The vBulletin Options > vBulletin Options > Date and Time Options setting group is used to define the way dates and times are displayed on certain sections of the board in what way and how they should behave.

Datestamp Display Option
This option controls the display of dates throughout your forum
'Normal' uses the date and time formats below this option.
'Yesterday / Today' will show 'Yesterday' and 'Today' for dates that fall in those periods.
'Detailed' will show times such as '1 Minute Ago', '1 Hour Ago', '1 Day Ago', and '1 Week Ago'.
Default Time Zone Offset
Time zone offset for guests and new users. Do not take DST into consideration, rather use the next option to enable/disable DST.
Enable Daylight Savings
If Daylight Savings Time is currently in effect for the above time zone, enable this option so that guests will see the correct times on posts and events. This has no effect on registered users as they control their DST options in the User CP. This setting is not automatic and it will need to be changed when the timezone changes twice a year.
Format For Date
Format in which the date is presented on vBulletin pages.

Examples:
US Format (e.g., 04-25-98): m-d-y
Expanded US Format (e.g., April 25th, 1998): F jS, Y
European Format (e.g., 25-04-98): d-m-y
Expanded European Format (e.g., 25th April 1998): jS F Y
Format For Time
Format in which the time is presented on all vBulletin pages.

Examples:
AM/PM Time Format (eg, 11:15 PM): h:i A
24-Hour Format Time (eg, 23:15): H:i
Format For Registration Date
This is used to format dates shown with users' posts. In the left hand column of a topic display, under the username and title, there is some text showing when the user registered.
Format For Birthdays with Year Specified
Format of date shown in profile when user gives their birth-year.
Format For Birthdays with Year Unspecified
Format of user's birthday shown on profile when the user does not specify their birth-year. DO NOT put in a code for the year.
Log Date Format
Format of dates shown in Control Panel logs.

Note:

Date and Time formats follow PHP formatting rules. You can find out more about these rules by visiting the PHP manual.
For information on date and time formats in PHP please visit the following page: http://www.php.net/manual-lookup.php?function=date

When you are done, click the [Save] button to apply the changes.