The exhortation is, "Today, if you hear his voice, do not harden your hearts as they did in the rebellion." Also, "Exhort one another every day, as long as it is called today". So when is Today?
This may seem like a silly question, but it is not, especially in light of yesterday's post. One interpretation is that, when one once hears God's voice, one must respond that day, or be cut off. One chance, one day. Because the next day is tomorrow, and today becomes yesterday. You miss out, it's too bad. You blew it.
Alternatively, the author is getting at the point that for us every moment is "today". Being bound into the flow of time, we can only ever act "today"; there is no real possibility of acting tomorrow, until it becomes today. I have made many plans for many tomorrows over the years that I failed to carry through when they became "today". It is what one does now that matters. The least important time is the past, wherein one can neither act nor plan. The less important time is the future, of which one knows little and can only plan. The most important time is the present, which one can know and in which one can act.
For us, it is always Today. And, as I suggested yesterday, the future, even the future as pronounced from time to time by God, is changeable, at least in the only way which we can understand. When we kneel before God and repent, the world changes. When we open our hearts to God today, regardless what we did yesterday, the world changes. Today is the only day in which we can act. Now is the only moment in which we can act. And as we are born along in time, every moment is new and therefore a new opportunity to enter the rest that God has offered us.