Commentary for Kilayim 7:1
בניך לא יהא בהן פסולת. הא מכלל שיש בהן פסולת. שנייא היא הכא שהוא עתיד למתקה כהדא רבי שמעון בר' הוה משקי פרסתקיה יין מבושל בשביל למתקה:
R. Aha said: It is written (in Gen. xliv. 3): "As soon as the morning was light." The Tori calls the light "morning." R. Ishmael taught: It is written "every morning," so as to give a limit for him who desires to know when the morning commences. R. Yosse bar R. Aboon said: If you think to call night, the time that the sun takes to traverse the heavens (from dawn to radiancy), it would be equivalent to saying that the day and the night do not resemble each other (the night would lengthen out to the morning by this addition; but we are taught that on the first day of the Equinox of Nissan ', and on the first day of the Equinox of Tissri, the day and the night are equal). R. Hoona says: One can accept the usual custom as a term of comparison. Thus, when the king starts to go out, he is said to be out; but when he commences to return, he is not said to be returned, until it is an accomplished fact (it is the same with the sun).