And another angel came and stood at the altar with a golden censer, and he was given much incense to offer with the prayers of all the saints on the golden altar before the throne, and the smoke of the incense, with the prayers of the saints, rose before God from the hand of the angel. Then the angel took the censer and filled it with fire from the altar and threw it on the earth, and there were peals of thunder, rumblings, a flash of lightning, and an earthquake. The censer was filled with fire, and it was thrown on the earth. The first of the calamities is that a third of the trees, etc., are burned in Rev. 8 and Rev. 16, and terrible sores will appear on those who take the mark. This is a spiritual event, not a natural one. 

These events happen during the time of tribulation, but do not last the full three and a half years. Matthew 24 states clearly that this time comes to an end, then there are some events, and then Christ returns. When it starts is not clear; the only thing we have to work with are scorpion-like creatures that are released from the pit and last for five months. Even though much of the mark of the beast system is in place, the Antichrist has to cause all people to take the mark. That is an action by him that will likely take at least a year. The first calamity hurts those who have taken the mark.