Our Sages said, ‘If not now, when?’. Included in this statement is that a person should not think that he can make up his learning time later. Hashem gives each person a daily assignment, and what he doesn’t do today will stay undone, because each day has its own assignment, and our Sages only allowed a person to fill in at night what he missed during the day, not from one day to the next. This may be included in the meaning of the verse, ‘Beware lest … [the words of Torah] be removed from your heart all the days of your life’, implying that one should watch each day lest he not learn Torah that day.