Time Units

Solidity provides some native units for dealing with time.

The variable nowwill return the current unix timestamp of the latest block (the number of seconds that have passed since January 1st 1970). The unix time as I write this is 1515527488.

Unix time is traditionally stored in a 32-bit number. This will lead to the "Year 2038" problem, when 32-bit unix timestamps will overflow and break a lot of legacy systems.

So if we wanted our DApp to keep running 20 years from now, we could use a 64-bit number instead — but our users would have to spend more gas to use our DApp in the meantime. Design decisions!

Solidity also contains the time units

• seconds

• minutes

• hours

• days

• weeks

• years

These will convert to a uint of the number of seconds in that length of time.

Here's an example of how these time units can be useful:

uint lastUpdated;

// Set `lastUpdated` to `now`
function updateTimestamp() public {
  lastUpdated = now;
}

// Will return `true` if 5 minutes have passed since `updateTimestamp` was 
// called, `false` if 5 minutes have not passed
function fiveMinutesHavePassed() public view returns (bool) {
  return (now >= (lastUpdated + 5 minutes));
}

uint id = zombies.push(Zombie(_name, _dna, 1, uint32(now + cooldownTime))) - 1;

The uint32(...) is necessary because nowreturns a uint256by default. So we need to explicitly convert it to a uint32.