Mastering Blockchain Programming with Solidity

Getting the current time using the now variable

The now variable provides the current block timestamp as a uint256 value. This is an alias for block.timestamp. 

You should not rely on the correctness of block.timestamp or now, because these timestamps can be manipulated by the miners to some extent. These should not be used to generate random numbers. Random numbers will be deterministic if you use timestamp values to try to generate randomness, and miners would be able to attack your contract.