Dephosphorylates CDKN1B at 'Thr-187', thus removing a signal for proteasomal degradation. H2O + O-phospho-L-seryl-[protein] = L-seryl-[protein] + phosphate H2O + O-phospho-L-threonyl-[protein] = L-threonyl-[protein] + phosphate Named isoforms=2. Belongs to the PP2C family.