Sforno on Deuteronomy 19:18/ספורנו על דברים י״ט:י״ח

1והנה עד שקר העד, שקר ענה באחיו. His testimony was deliberately false, meant to incriminate his fellow Jew. He did not err in what he thought he had witnessed, such as there having been poor lighting on a cloudy day. Neither did he err in the day of the month when the supposed crime occurred, having been unaware that the most recent month had been extended by a day.