I belong to an exclusive group that has needed to write software that calculates the time span between two (local) instants in time. And now, we can't just use

I belong to an exclusive group that has needed to write software that calculates the time span between two (local) instants in time. And now, we can't just use GMT. The inputs we get are local time, by specification. I have strong opinions about DST. And even more fringe-y, I have opinions about time zones too.