The viewer shows the time of day in at least one place, that being the right end of the menu bar. There it shows time in either 12-hour or 24-hour format, and either PST (Pacific Standard Time) or PDT (Pacific Daylight Time). The viewer may also show timestamps in chat and the Conversation Log; those are also PST or PDT and are always 24-hour format. And if you show timestamps for Teleport History, that is UTC/GMT (7 or 8 hours before SLT, depending on Daylight Savings) and is in 24-hour format.
If your computer's default language is English then it will show 12-hour format; French and German, among others, will show 24-hour time. But if your computer does not have a default language installed, the viewer will show 24-hour format. There are two ways to resolve this:
Remember to reboot your computer if you change its configuration, and restart Firestorm if you change a viewer setting.