org-time-stamp-rounding-minutes

org-time-stamp-rounding-minutes is a variable defined in `org.el'.
Its value is

(0 5)

Documentation:
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.

When a setting is 0 or 1, insert the time unmodified. Useful rounding
numbers should be factors of 60, so for example 5, 10, 15.

When this is larger than 1, you can still force an exact time stamp by using
a double prefix argument to a time stamp command like `C-c .' or `C-c !',
and by using a prefix arg to `S-up/down' to specify the exact number
of minutes to shift.

You can customize this variable.