Basic Solidity contract to tell the seconds past after update function calls.

StopWatch.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract StopWatch {
    uint256 lastUpdateTimestamp;

    constructor() {
        lastUpdateTimestamp = block.timestamp;
    }

    function getSecondsSinceLastUpdate() public view returns(uint256) {
        return block.timestamp - lastUpdateTimestamp;
    }

    function update() public {
        lastUpdateTimestamp = block.timestamp;
    }
}