Musar zu Mischlej 3:28

And, in truth, "And if not now" implies "every hour and every day." One must think "If not now" — that the Holy One Blessed be He determines for a man every day what he is to attain of the Torah of the L-rd and His service. As to the yetzer's enticing him that tomorrow he can make up for today, this is an error. For tomorrow is an obligation in itself, and he will not be able to make up the time that he lacks now. And only from day to evening [of the same day] do we find in Chazal: "One may borrow [of his time] in the daytime and make it up [in Torah study] at night." And this is the intent of (Devarim 4:9): "And lest they depart from your heart all the days of your life." And thus, (Koheleth 9:9): "And see life… all the days of your vanity" — that is, that not one day go lost. And thus, (Rambam, Hilchoth Talmud Torah 1:10): The verse "And lest they depart from your heart all the days of your life" implies that one may not be idle in the Torah of the L-rd for even one day — until his last day. And this is intimated in the verse (Mishlei 3:28): "Do not say to your friend: 'Go and return, and tomorrow I will give you,' if you have it with you [today]." "your friend" is the good yetzer, a man's true friend, who counsels him as to what will be good for him forever.