Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
inter-sto, stĕti, 1, v. n. and a., to stand or be between or in the midst of (post-class.): tempus interstat, Amm. 22, 11, 3; 16, 9, 4.
With acc., Avien. Perieg. 849.