And if not now, then when? (Ethics of the Fathers 1:14).
Hillel's famous statement is a bit enigmatic. The simple answer is, "Later." Why can't we take care of whatever it is some other time? Granted that procrastination is not a virtue, why does Hillel imply that if not now, then it will never be?
The Rabbi of Gur explained that if I do something later, it may indeed get done, but I will have missed the current "now." The present "now" has but a momentary existence, and whether used or not, it will never return. Later will be a different "now."
King Solomon dedicates seven famous verses of Ecclesiastes to his principle that everything has its specific time. His point comes across clearly: I can put off doing a good deed for someone until tomorrow, but will that deed, done exactly as I would have done it today, carry the same impact?
The wisdom that I learn at this moment belongs to this moment. The good deed that I do at this moment belongs to this moment. Of course I can do them later, but they will belong to the later moments. What I can do that belongs to this moment is only that which I do now.