ERC20Lockable.sol

pragma solidity ^0.5.0;

import "http://github.com/OpenZeppelin/openzeppelin-solidity/contracts/token/ERC20/ERC20.sol";

contract ERC20Lockable is ERC20 {
    
    mapping (address => uint) unlockTime;
    mapping (address => bool) isLocked;
    uint lockDuration = 1000;
    
    modifier isNotLocked(){
        if(isLocked[msg.sender]){
            require(unlockTime[msg.sender] >= now, "Tokens are still locked.");
            isLocked[msg.sender] = false;
        }
        _;
    }
    
    function lock() public
    {
        //require(isLocked[msg.sender] == false, "Tokens are already locked");
        unlockTime[msg.sender] = now + lockDuration;
        isLocked[msg.sender] = true;
    }
    
    function transfer(address recipient, uint256 amount) public isNotLocked() returns (bool) {
        _transfer(_msgSender(), recipient, amount);
        return true;
    }
    
}