Albert Barnes on Hebrews 3:15

While it is said, To-day, etc. That is, persevere as long as life lasts, or as long as it can be said "To-day;" and by persevering in this manner you will have evidence that you are the friends of the Redeemer. This is a quotation from Ps 95:7. Paul means, undoubtedly, to make use of this language himself as a direct exhortation to the Christians to whom he was writing. He entreats them, therefore, as long as it could be said "To-day," or as long as life lasted, to take care lest they should harden their hearts, as had been done in the temptation in the wilderness.

(b) "Today" Heb 3:7