Tvheadend

Is this a bug ? (I don't understand the purpose.).

The only purpose I can think of is if someone wanted to record a segment of a bigger transmission. So one would set "start recording x minutes after the start of the transmission" and "stop recording y minutes before the end". The problem would be that by doing that you could come up with a negative recording duration if you set the x and y values wrong.
I agree this is not a very useful feature, if that's the original request.
Usually people set a bigger recording time and then, if necessary, cut the parts in excess using some "post-processing" tools.

It is not a bug but feature
I think it would be useful for skipping commercial
It is very common epg starttime include commercial time before main program

i was also searching for such a feature
it would be nice to have a "negative padding"