“Hillel said, ‘If not now, when?’” (Pirkei Avos 1:14)

The plain meaning is that if you push off doing something positive, you might end up not doing it at all.  New obstacles may arise and prevent you from ever doing it.

But we can understand Hillel another way: “Very good, we accept that you truly will do the good deed later on, but what about the present?” You must do all you can both now and later. Even though you might utilize the opportunity to do good later on, you have, nonetheless, wasted the present moment.

~ R’ Chaim Mordechaim Katz zt”l (1894-1965)