AIX

In releases prior to Domino 6 for AIX, Solaris, Linux, iSeries and zSeries platforms, the clock type used by the Domino server defaulted to 12 hour format. On Domino 6, the default clock type is inherited from the operating system date/time format configuration.
The Domino 6 server inspects a locale keyword named "d_t_fmt" in a category "LC_TIME" to decide the default clock type. As a result, you see the time format used by the Domino server has changed from 24 hour format (military) to 12 hour format after upgrading.

Solution
If you desire to retain the clock type as 12 hour, you can do one of the following:
Change the locale to another which uses 12 hour format for starting Domino 6 server. For example, a locale "en_US" on Linux and Solaris platforms uses 12 hour clock.
Specify the notes.ini setting "ClockType=12_HOUR" to override the system default clock type.