1. day, i.e., a unit of time reckoned from sunset to the next sunset, including two or more segments (morning and evening) about 24 hours (Ge 1:5), cf. also 3429; 2. LN 67.163–67.200 day, i.e., the period of time which has light (Ge 1:5)
-James Swanson, Dictionary of Biblical Languages With Semantic Domains : Hebrew (Old Testament), electronic ed. (Oak Harbor: Logos Research Systems, Inc., 1997).
2. day of twenty-four hours: Gn 1:5
-Ludwig Koehler, Walter Baumgartner, M.E.J Richardson and Johann Jakob Stamm, The Hebrew and Aramaic Lexicon of the Old Testament, electronic ed. (Leiden; New York: E.J. Brill, 1999), 399.
I've consulted many sources, but was unable to find even one that defined "day" in Genesis 1 to refer to anything other than a 24 hour day. If anyone has any lexical sources which site to the contrary, i'd be interested to look at it.