You can manipulate a timestamp relatively easily since you are submitting work to a network that does not know when you did that work. That being said they CAN for example ensure that you are not setting a time that has not happened yet. They can also ensure that your timestamp falls in between the acceptable window for solution times based on previous block times. Since this window is relatively large compared to the average block time, "pre-mining" works would be highly improbable for anyone to pull off since they would essentially need to solve the block before everyone else on demand and then submit them delayed which decreases their chance for propagation.