John Gill Reformed Baptist @doctorgill

Again he went out about the sixth and ninth hour,.... About twelve o'clock, or at noon, and three o'clock in the afternoon. These three last mentioned seasons of the day, were the hours of prayer; see Act 2:15 and did likewise: seeing others in the same place, and posture, he called them, and sent them into his vineyard, to labour there, giving them the same promise he did to others.

Albert Barnes Presbyterian @notesbybarnes

Verse 5. The sixth and ninth hour. That is, about twelve and three o'clock.

Jamieson-Fausset-Brown Reformed @jfbcommentary

Again he went out about the sixth and ninth hour--about noon, and about three o'clock in the afternoon. and did likewise--hiring and sending into his vineyard fresh laborers each time.