How to implement time based decisions into a contract

Contracts live in their own world clocked by block times. Sometimes the real world still requires tribute by using human clocks and timestamps.

Examples can be a mint start at a specific time or a functionality available during a defined timeframe.

Calculating a timestamp based on the block numbers can be a solution but block times can vary depending on network usage or even outages.

Global block

Within Solidity access to the human clock is available using the global variable block.

block.timestamp contains the timestamp of the current block.

Here is an example with a simple countdown that shows the seconds left until the defined countdown:

uint256 public timestamp;
uint256 public changedAt;
 
function setCountdownTo(uint256 timestamp_) public {
    timestamp = timestamp_;
    changedAt = block.timestamp;
}
 
function countdown() public view returns (uint256 secondsLeft) {
    secondsLeft = (timestamp - block.timestamp);
}

Time calculation

Time calculations are made additional easy using time units that allow a human readable conversion.

To output the countdown in minutes or days, it can be divided:

minutesLeft = (timestamp - block.timestamp) / 1 minutes;
daysLeft = (timestamp - block.timestamp) / 1 days;
hoursLeft = (timestamp - block.timestamp) / 1 hours;
weeksLeft = (timestamp - block.timestamp) / 1 weeks;
yearsLeft = (timestamp - block.timestamp) / 1 years;

Read more at the Solidity docs: https://docs.soliditylang.org/en/v0.4.21/units-and-global-variables.html#time-units (opens in a new tab) The example contract is available here: https://gitlab.com/vechain.energy/examples/contract-timer (opens in a new tab)