1. Start ==> Programs ==> Oracle Content Server ==> idc ==> Utilities ==> System Properties, click Localization tab, click English-US (or whatever your default locale is), click Edit..., edit Date/Time Format entry, you can format there your time to 24 hours if you use the appropriate formatting.
The normal convention for 24h is: HH:mm:ss, change HH to hh for 12h.
What exactly do you mean where do you want to see the time in 24hr format?
You would do well to read about SystemTimeZone SystemLocale SystemDateFormat and formatDateWithPattern in the Idoc script reference http://download.oracle.com/docs/cd/E10316_01/cs/cs_doc_10/documentation/developer/idoc_script_reference_10en.pdf
The IDOC script fucntion dateCurrent will give you the full date and time 'now' and you can show this in the time only format like this <$formatTimeOnly(dateCurrent())$>
In terms of setting this for dates selected by calendar...
You can either do it server side by setting up a derived rule where you could change the time after checkin to 'now' using the above
Or you could look at customizing the PopUpCalendar resources. In this case you would need to modify the resource so that when the page was rendered the time part was stored and that on selection of a data from the calendar the time is cocatenated on the end. You would need to do this in a component of course.
The second solution is more work but would look more elegant