If the half hour of silence starts when the seventh seal opens and ends when Christ comes at the Second Coming it can’t be measured on earth’s clock. I see two reasons to believe it starts when the seventh seal opens and ends when Christ comes.
1. In Rev. 8 vs 1 it says there is a half hour of silence when the seventh seal opens. In D&C 88 vs 95 it says the face of the Lord is unveiled immediately after the half hour of silence. There are either two half hour of silences measured on earth’s clock, or just one that must be measured on heavens clock (about 21 of our years).
2. Saying that the silence is caused by wickedness is the same as saying the half hour of silence doesn’t end until the Second Coming.

Since the half hour of silence infers the wickedness that prevails when the seventh seal opens, it explains why the angel with the golden censer takes the prayers of the saints to heaven and returns with destruction for the wicked. The saints must be praying for deliverance from the wicked.

Perhaps there are some clues that say how the 21 years are divided up. The Gog & Magog siege of Jerusalem takes 3.5 years. The burning of the weapons of war after the Gog & Magog war takes 7 years. The woman (the church) is forced into the wilderness (possibly places of refuge) for 3,5 years when the seventh seal opens. That leaves 7 years after the woman comes out of the wilderness to build New Jerusalem before the Gog & Magog war begins.

The Jews must have a period of peace after the initial destructions of the seventh seal. Gog & Magog invade Isreal when they are dwelling safely in unwalled cities. Even Gog must take a while to recover from destructions that kill a third part of man.

If wickedness causes silence to reign, and wickedness increases as the Second Coming draws nearer, we should expect silence to reign from the opening of the seventh seal until the Second Coming.

Silence must have reigned at Noah’s and Enoch’s time also. In the P of GP we are told that the wicked were deaf to the voice of the Lord and that they were shut out from the presence of the Lord.

Obviously, silence in heaven only applies to the wicked. In Psalms 50:3 we are told that God breaks HIs silence at the Second Coming when he comes to destroy the wicked who caused the silence:

“Our God shall come, and shall not keep silence: a fire shall devour before him, and it shall be very tempestuous round about him.” (Psalms 50:3)
Using this description, silence in heaven is not a time of inactivity in heaven. Instead it symbolizes the polarization between the wicked and the righteous. In D&C 38:12 it even says that “the enemy (the wicked) are combined.”
I also means that the angels are not waiting for the silence in heaven to be over before they receive the command to reap down the earth. They are waiting for silence to begin.