It is not a shortening of the hours of the days, but rather a shortening of the light and heat of the sun.

In Revelation 16:8-9, …the fourth angel poured out his vial upon the sun, s-u-n, and power was given unto the sun to scorch men with fire and men were scorched with a great heat... It’s a judgment that comes during the tribulation hour because of sin and they wouldn’t repent. Carl Sagan once said there’s an hour coming when there’s going to be a horrendous earthquake and it takes place and blots out the sun. And that is God’s way of protecting people so that they won’t all die through that judgment that’s hitting the earth. And that’s there in Revelation 6:12, …I beheld when he opened the sixth seal and lo there was a great shaking, a great earthquake, and the sun became black as sackcloth of hair and the moon became as blood. Darkened, and he said that could last for 90 to 120 days. But Jesus said in Matthew 24:29, that immediately after the tribulation of those days, at the end of the seven year period before Christ comes to set up his kingdom on earth forever he said the sun and the moon were darkened. Why? To save life.