FR: populate default prompt text for org-time-stamp on an existing time stamp?

[Adam Spiers <orgmode@adamspiers.org>]

Another feature request (sorry - I'm beginning to feel quite guilty
about the deluge I've rained on Carsten's inbox recently :-)

If I position the point on an existing timestamp which includes not
only the date but also the time, e.g.

* <2007-11-27 Tue 19:30-20:30> 

and then hit 'C-c .', then the popup date chooser nicely puts the
point on November 27th in the Calendar buffer.  However, the existing
time is ignored, which means if I want to tweak the start or end
times, I have to retype something like "7pm+1".  It would be really
nice if the minibuffer prompt was automatically populated with "7:30pm+1"
so that for example shifting the whole appointment 30 mins earlier
would be a simpler matter of changing a '3' digit to a '0'.