> For example I am working on project A from 09:00 until 13:00. But

> during this period I also worked for fifteen minutes on project B. I
> would like to subtract those fifteen minutes from project A and add
> them to project B. Can this be done?

I assume you stopped the clock on A, then started it on B, then
stopped it on B, then restarted it on A.

In this case, go to the end of the B clock timestamp, put the point
on the HH:MM string, and hit S-M-<down> three times, so that the clock
sum for B is zero, and the clock sum for A is added 15 minutes.