ISP:
Telenet (but that's rather irrelevant)

Type of Internet Connection:
Cable (irrelevant)

Internet Connection Speed:
~5Mbit/s

Date & Time:
All the time

Frequency:
Always

Character Name:
All of them (irrelevant)

Race:
All (irrelevant)

World:
All (irrelevant)

Class/Level:
All (irrelevant)

Area and Coordinates:
All (irrelevant)

Party or Solo:
Always (irrelevant)

NPC Name:
None

Monster Name:
None

In-Game Time:
Always

Steps:
(windows version)
Go to System Configuration -> Log Windows Settings
Check Add time stamp to message
Put "Time Display" on "24-hour Format"
It will always display in "12-hour Format" regardless of the setting


PS: I searched, but couldn't find a related bug-report