Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
per-dĭŭturnus, a, um, adj., that lasts a very long time, very lingering or protracted (rare but class.), Cic. N. D. 2, 33, 85 (but Cic. Sest. 27, 58 Halm and B. and K. read diuturnum).