Principal English Translation: 

by day; day

Alonso de Molina: 

tlacatli. dia.
Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 115v. col. 2. Thanks to Joe Campbell for providing the transcription.

Frances Karttunen: 

TLAHCAH-TLI daytime, midday / día, desde que el sol sale hasta que se pone (C), mediodía (T). Without the absolutive suffix TLAHCAH commonly means ‘at midday’, while with the absolutive suffix it refers to the middle of the day itself. See TLAHCO.
Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 260.

Lockhart’s Nahuatl as Written: 

nominalized particle. the daytime, the central part of the day after early morning until into the afternoon. tlahcah.
James Lockhart, Nahuatl as Written: Lessons in Older Written Nahuatl, with Copious Examples and Texts (Stanford: Stanford University Press and UCLA Latin American Studies, 2001), 236.

Attestations from sources in English: 

ye tlaca = it's late

Robert Haskett and Stephanie Wood's notes from Nahuatl sessions with James Lockhart and subsequent research.