That actually depends on your system time format. Plus! uses the format that your Windows is using. So if you want a 24h clock, change your Windows to use the 24h clock in the config panel ("local settings", or "country settings" or something (I don't have a English OS, sorry))
But you would notice that the window where the time is shown is still a bit small and the available space isn't used that well: big gap between the time and the event...
Patchou has said in the past that he would improve the contact info window to address this issue, but I guess he forgot about it
hint hint
eg:
http://shoutbox.menthix.net/showthread.php?tid=27351