If you cannot type new event time in 24 hour format in Entourage 2008 (and your system settings are OK in International), try to install latest Leopard 10.5.2 update. This worked for me, issue is gone.
I already have Leopard 10.5.2, my system settings under international are OK), my computer displays my time in 24 hour format (currently 18:36). I still cannot get 24 hour format in Entourage. Any more ideas?
