Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
ad-vespĕrascit, āvit, 3, v. impers. and inch., it approaches evening, it is getting to be evening, twilight is coming on: advesperascit, Ter. And. 3, 4, 2; Vulg. Luc. 24, 29: cum jam advesperasceret, Cic. Verr. 2, 4, 65, etc.; id. Fin. 4, 28: nisi advesperāsset, Auct. B. Hisp. 24: cum advesperavisset, Plin. 7, 52, 53, § 178: advesperascente die, Vulg. Prov. 7, 9.