'Harden Your Hearts' in the Bible

Do not harden your hearts as your forefathers provoked me by doing, As in that time of trial in the desert,

So while we can still speak of Today, if you hear him speak, do not harden your hearts, as they provoked him by doing.

he again fixes a new Today, saying long afterward through David, as already quoted, "If you hear his voice today, Do not harden your hearts!"