That being said they CAN for example ensure that you are not setting a time that has not happened yet (although this still is not a reason for concern).
The timestamp of a block can be up to 2 hours ahead of the current adjusted network time.
come to think of it I cannot think of a practical reason that anyone would want to manipulate a timestamp on bitcoin
There are a number of potential attacks which involving manipulating the timestamp to artificially lower the difficulty and therefore allow malicious miners to mine blocks more quickly. While such attacks have been seen on altcoins, they've never been seen on bitcoin as they would be prohibitively expensive.