you 've spent the past 20 minutes looking at your watch 
(ROOT (S (NP (PRP you)) (VP (VBP 've) (VP (VBN spent) (NP (DT the) (JJ past) (CD 20) (NNS minutes)) (S (VP (VBG looking) (PP (IN at) (NP (PRP$ your) (NN watch)))))))))
(ROOT (S (NP (PRP )) (VP (VBP ) (VP (VBN ) (NP (DT ) (JJ ) (CD ) (NNS )) (S (VP (VBG ) (PP (IN ) (NP (PRP$ ) (NN )))))))))