title: Avoid created_at and updated_at being auto generated by sequelize
body: <p>How do I define a model for which created_at and updated_at are provided rather than generated?</p>
  
  <p>I'm importing data from somewhere that already has data for <code>created_at</code> and <code>updated_at</code> fields that I would like to preserve rather than generating whenever the object is created/updated by sequelize (our secondary store).</p>
  
  <p>I've tried every likely permutation of model definition and options to get this to work and still sequelize overwrites my fields with it's own timestamps: <code>{silent: true}</code>, etc...</p>
  
  <p>To be clear, the input data has createdAt and updatedAt and I'd like to use sequelize's bulkCreate(), etc such that input values provided are used and stored on the model as created_at and updated_at rather than generated by sequelize.</p>
  
  <p>This is my current model definition:</p>
  
  <pre><code>const Lead = sequelize.define('lead', {
    objectId: {
      type: Sequelize.STRING,
      field: 'objectId',
      primaryKey: true,
      allowNull: false
    },
    firstName: {
      type: Sequelize.STRING,
      field: 'first_name'
    },
    lastName: {
      type: Sequelize.STRING,
      field: 'last_name'
    },
    phoneNumber: {
      type: Sequelize.STRING,
      field: 'phone_number'
    },
    createdAt: {
      type: Sequelize.DATE,
      field: 'created_at',
    },
    updatedAt: {
      type: Sequelize.DATE,
      field: 'updated_at'
    }
  }, {
    freezeTableName: true, // Model tableName will be the same as the model name
    timestamps: false,
    underscored: true
  });
  </code></pre>