title: sequelize - how to set validate rule for Date field
body: <p>I have a DATE field called <code>completedAt</code>, which should only accept the value on or after the current datetime.</p>
  <p>I think I have to add a validate rule on <code>completedAt</code>, but I don't know how to add the condition</p>
  <pre><code>const Sequelize = require('sequelize');
  const DataTypes = Sequelize.DataTypes;
  
  module.exports = function (app) {
    const sequelizeClient = app.get('sequelizeClient');
    const homework = sequelizeClient.define('homework', {
      ...,
      completedAt: {
        type: DataTypes.DATE,
        validate: {//What should I do here}
      },
    }, 
    });
  
    homework.associate = function (models) {
    };
  
    return homework;
  };
  
  </code></pre>