"If not today, then when?" is a powerful reminder and call to action. The future is mostly unpredictable. We cannot be sure of what may happen later today, tomorrow, next week, next month or next year. The future is never the present. It is always the future. We should always live in the present.
Now is the time to start. It is almost never a good time to start something. We can always procrastinate and plan to start. There's almost never a perfect time to do anything. Now is the time!