Solidity – Time Dependent Contract Testing in Hardhat

I am trying to test a contract in which every 4 minutes the price of token is changing.
But I am not able to test the contract as I am unable to manipulate the time.The test without time manipulations are running fine but I have no idea about the time manipulation in hardhat. please help.
Here is the code.

const { expect } = require("chai");
const { ethers } = require("hardhat");
const {helpers}= require ("@nomicfoundation/hardhat-network-helpers");
console.log(mine)

describe("Token contract", function () {
    it("Deployment should assign the total supply of tokens to itself", async function () {
        const [owner] = await ethers.getSigners();

        const Token = await ethers.getContractFactory("Token");

        const token = await Token.deploy();

        const contractBalance = await token.balanceOf(token.address);
        expect(await token.totalSupply()).to.equal(contractBalance);
    });

    it("buyer can buy the token", async function () {
        const [owner, buyer] = await ethers.getSigners();
        const Crowdsale = await ethers.getContractFactory("Crowdsale");
        const crowdsale = await Crowdsale.deploy();

        const Token = await ethers.getContractFactory("Token");
        const token = await Token.deploy();

        await crowdsale.setTokenAddress(token.address);
        await token.approveContract(crowdsale.address);
        await crowdsale.setStart();

        await crowdsale.connect(buyer).buyToken({ value: ethers.utils.parseEther("1") })
        expect(await token.balanceOf(buyer.address)).to.equal("1000000000000000000000")
    })

    it("Prices should change accoording to time", async function () {
        const [owner, buyer] = await ethers.getSigners();
        const Crowdsale = await ethers.getContractFactory("Crowdsale");
        const crowdsale = await Crowdsale.deploy();

        const Token = await ethers.getContractFactory("Token");
        const token = await Token.deploy();

        await crowdsale.setTokenAddress(token.address);
        await token.approveContract(crowdsale.address);
        await crowdsale.setStart();
        
        await helpers.time.increase(240);

        await crowdsale.connect(buyer).buyToken({ value: ethers.utils.parseEther("1") })
        expect(await token.balanceOf(buyer.address)).to.equal("1000000000000000000000")

        await helpers.time.increase(240);
        await crowdsale.connect(buyer).buyToken({ value: ethers.utils.parseEther("1") })
        expect(await token.balanceOf(buyer.address)).to.equal("500000000000000000000")

        await helpers.time.increase(240);
        await crowdsale.connect(buyer).buyToken({ value: ethers.utils.parseEther("1") })
        expect(await token.balanceOf(buyer.address)).to.equal("200000000000000000000")



    })


});

Here is the error.