'Harden' in the Bible

do not harden your hearts as your forefathers did in the time of the provocation on the day of the temptation in the Desert,

seeing that the warning still comes to us, "To-day, if you hear His voice, do not harden your hearts as your forefathers did in the time of the provocation."

He again definitely mentions a certain day, "To-day," saying long afterwards, by David's lips, in the words already quoted, "To-day, if you hear His voice, do not harden your hearts."