Negative SMPTE timecode drop-frame handling?
I know this is a very old Q, but it should be pointed out that a 'negative timecode' makes no sense. The SMPTE frame before 00:00:00:00 is 23:59:59:29. The namespace for drop frame (and non-drop) codes is already well-defined -- no need to postulate negative values. If there are systems or programs that use this notation, they should be rethought.