They might not even be trying to "stretch" it (a large company could give a crap about the float on an indie consulting invoice); they might just run accounts payable on a state machine in which the part that elapses a timer hasn't started yet. There might be all manner of reasonable reasons for doing it that way; for instance, if they accidentally release payment early, they may screw up projects by blowing expectations.
In any case, yeah: some of your best clients will do a terrible job paying you on time. 'slife in the big leagues.