Matthew 8:22 But Jesus said unto him, Follow me; and let the dead bury their dead.
Its impossible for the physically dead to bury the physically dead. So we know that's not what Jesus meant by saying, "let the dead bury the dead."
But it is possible for the spiritually dead (dead in sins) to bury the physically dead. Therefore that's the answer as to who the "dead" are in this verse. It is the spiritually dead-
Ephesians 2:1 And you hath he quickened, who were dead in trespasses and sins;
-who are physically alive who are able to bury those who have died physically.
Therefore, when Revelation 11:18 tells us that it is "the time of the dead" that they should be judged-we should not immediately assume it is speaking of God judging those who have physically died. Actually, the time of God's judging of the dead is happening now as He pours out His wrath (spiritually) upon all unsaved (spiritually dead) people of the world.
"Well, if that were true," someone might say, "what are we (the saved) doing here?" The answer to this is question is found in:
2 Timothy 4:1 I charge thee therefore before God, and the Lord Jesus Christ, who shall judge the quick and the dead at his appearing and his kingdom;
God-who is presently judging the dead-is also judging the quick. The quick are those made alive through God's salvation.
Ephesians 2:5 Even when we were dead in sins, hath quickened us together with Christ, (by grace ye are saved;)
We are all (saved and unsaved; quick and the dead) present together on the earth to experience, or go through the prolonged judgment period known as the "time of the dead" that they should be judged.
Further on, in the same chapter that spoke of a time for everything, a time to live and a time to die, we read:
Ecclesiastes 3:16 And moreover I saw under the sun the place of judgment, that wickedness was there; and the place of righteousness, that iniquity was there.
17 I said in mine heart, God shall judge the righteous and the wicked: for there is a time there for every purpose and for every work.
God shall judge the righteous (the quick) and the wicked (the dead)-for THERE IS A TIME there for every purpose and for every work.