When I create an appointment in org-agenda I make use of timestamps. The item is created with today's date and time and I then place my cursor on the date and time respectively and use S-up or S-down to increase or decrease the time until I get to the time I need. I see that time increments by default are 5 minutes. I would prefer it to be 30 minutes. Can can I achieve this?
S-up and S-down are bound to
org-shiftdown. On timestamps, these call
org-timestamp-down respectively. These methods use the variable
Number of minutes to round time stamps to.
These are two values, the first applies when first creating a time stamp.
The second applies when changing it with the commands ‘S-up’ and ‘S-down’.
When changing the time stamp, this means that it will change in steps of N minutes, as given by the second value.
So we can change the second value to
(setf (elt org-time-stamp-rounding-minutes 1) 30)
And now these methods will move by 30 minutes.
 It will move to the nearest multiple of this value; S-up once on
<2019-10-09 Wed 06:27> will move it to
<2019-10-09 Wed 06:30>; calling it again gives
<2019-10-09 Wed 07:00>.