Smart contract now vs javascript now

Question

I have a trouble with comparing timestamp parameter(in test) and now in smart contract.

Here is my smart contract:

function create(uint created_at) public {
    require(now > created_at, "The created_at should be less than now .");
}

And test function:

it("...should store the created_at", async () => {
    const storageInstance = await Storage.deployed();
    let created_at = new Date().getTime();//create utc now timestamp 
    await storageInstance.create(created_at);
 }

The test doesn't pass. Error:

The created_at should be less than now .

Answer 1

now in Solidity is the amount of seconds since the Epoch, (see Is the block.timestamp value in Solidity seconds or milliseconds?). Javascript's new Date().getTime() / Date.now() returns the amount of milliseconds since the Epoch (see Date.prototype.getTime & Date.now).

This means that Javascript's timestamp is always a factor 1000 larger than the one used by Solidity. You could change your JS code to divide the timestamp by 1000.

let created_at = Math.floor(Date.now() / 1000)

Also be mindful of goodvibration's comment in your design.

UPDATE: new Date().getTime() --> Date.now() (goodvibration's comment)

Answer 2

I would avoid using javascript dates when working with Solidity. It's better to work with unix time directly. I've also been struggling with this and came up with this solution:

• use hardhat's network-helpers library to get the timestamp of the latest block
• use unix seconds to assert results
• use assert.closeTo() instead of assert.equalTo(), this way you can factor in you code execution time

Here I've made a simple example where you have a contract which sets a variable to next week when setNextWeek() is called. In the test we assert if the setNextWeek() function indeed set the time to next week.

Solidity

uint256 nextWeek;

function setNextWeek() public {
    nextWeek = block.timestamp + 1 weeks;
}

Hardhat Test

const helpers = require("@nomicfoundation/hardhat-network-helpers")


const ONE_WEEK_UNIX = 604800 // 1 week in UNIX time (seconds)

it.only("setNextWeek should set UNIX time to 1 week from now", async function () {
    await contract.setNextWeek()
    const nextWeekResult = await contract.nextWeek()

    const timestamp = parseInt(await helpers.time.latest());
    const nextWeek = timestamp + ONE_WEEK_UNIX
    
    // checks if the result is close to the expected result with a maximum difference of 5 (seconds)
    assert.closeTo(nextWeekResult.toNumber(), nextWeek, 5);
}