Timestamp Dependence
NOTE: This vulnerability no longer affects Ethereum mainnet as of the Proof of Stake merge. Read more
Timestamp Dependence is a vulnerability where the timestamp of a block, accessed by block.timestamp or alias now can be manipulated by a miner. There are three considerations you should take into account when using a timestamp to execute a contract function.
Timestamp Manipulation
If a timestamp is used in an attempt to generate randomness, a miner can post a timestamp within 15 seconds of block validation, giving them the ability to set the timestamp as a value that would increase their odds of benefitting from the function.
For example, a lottery application may use the block timestamp to pick a random bidder in a group. A miner may enter the lottery then modify the timestamp to a value that gives them better odds at winning the lottery.
Timestamps should thus not be used to create randomness. See Weak Sources of Randomness for Chain Attributes.
The 15-second Rule
Ethereum's reference specification, the Yellow Paper, doesn't specify a limit as to how much blocks can change in time, it just has to be bigger than the timestamp of it's parent. This being said, popular protocol implementations reject blocks with timestamps greater than 15 seconds in the future, so as long as your time-dependent event can safely vary by 15 seconds, it may be safe to use a block timestamp.
Don't use block.number as a timestamp
You can estimate the time difference between events using block.number and the average block time, but block times may change and break the functionality, so it's best to avoid this use.
Sources
https://consensys.github.io/smart-contract-best-practices/attacks/timestamp-dependence/
https://consensys.github.io/smart-contract-best-practices/development-recommendations/solidity-specific/timestamp-dependence/