"Remain until this very day, " doth not mean the day of the Reader's perusal of the history: but the day of the history itself being recorded.
Related Commentaries of Joshua 10:26
Joshua 10:26-27
26 And afterward Joshua smote them, and slew them, and hanged them on five trees: and they were hanging upon the trees until the evening.
27 And it came to pass at the time of the going down of the sun, that Joshua commanded, and they took them down off the trees, and cast them into the cave wherein they had been hid, and laid great stones in the cave's mouth, which remain until this very day.