I see this is filed as
RFE 24365 and is still open four years later.
I've posted how I'm working around this as a comment to that bug. Copy here:
This requires hand-editing a file and you will need to redo this whenever you
upgrade Zimbra.
This applies to Zimbra 5 and affects all users. I haven't (yet) tested this on
Zimbra 6.
cd /opt/zimbra/jetty/webapps/zimbra/
js
vi Calendar_all.
js (back it up first)
search for where the HOUR_HEIGHT variable is defined, it should be the topmost
result. It is ZmCalColView._HOUR_HEIGHT = 42 by default.
Change this to a multiple of 42 (ie 84 to give 15-minute increments) and save.
cp ZimbraMail_all.
js ZimbraMail_all.
js.
gzip -S zgz ZimbraMail_all.
js.
~/libexec/zmfixperms
zimbra stop; zimbra start
You will need to clear your (users') browser cache for it to show.