I remebered this issue has been dealt with in the tips and tricks for the S7-200. I knew I had them lying around somewhere, but in the end had to google the net for it, because I couldn't find them back. It's tip 31, which can be found here. This tip deals with off-delay timers, pulse timers and extended pulse timers, all of which are not present in the older S7-21X PLCs.