When editing a wait node it lets me put in a negative number for the wait time. Unfortunately my AI does not travel back in time and undo his previous actions like I hoped. Until time travel in UE4 is implemented, it is probably best to not allow negative numbers in this field.
There must be a bug somewhere since I’m pretty sure I did implement time-traveling for AI! It also included streaming navmesh from the future to cut navmesh generation time. My best guess is some low-level engine change broken my assumptions 
I don’t think I’ll be able to fix it for 4.7 
But thanks for reporting it! 
Cheers,
–mieszko