Mastering Blockchain Programming with Solidity

The 15-second blocktime rule

According to the yellow paper of Ethereum, the timestamp for the new block must be greater than the previous block; otherwise, the block will be rejected. The timestamp can be manipulated by the miners using the client software they are running to mine the blocks. There are two Ethereum client software used mostly, and these are Geth and Parity clients. Using their software code and algorithms, these two clients maintain the 15-second rule such that the timestamp difference between the two blocks should not be more than 15 seconds. Otherwise, the client software rejects the block.

Hence, if you are using block.timestamp or now in your contract code, you need to ensure that you are not performing any processes ...