Is there a simple way to make Sequelize return it's date/time fields in a particular format?

You can, use the Sequelize fn method. From the API Reference, the fn function will help create an object representing a SQL function in your query.

For example:

model.findAll({
  attributes: [
      'id',
      [sequelize.fn('date_format', sequelize.col('date_col'), '%Y-%m-%d'), 'date_col_formed']
  ]})
  .then(function(result) {
    console.log(result);
  });

Will return data values:

[
  {"id": 1, "date_col_formed": "2014-01-01"},
  {"id": 2, "date_col_formed": "2014-01-02"}
   // and so on...
]

21/06/2019

A little bit late but providing an update.

Sequelize is a powerful ORM (I am not saying is the best solution out there) but has a very bad documentation.

Anyway if you want to have this configured in your models one way apart from having to repeat this across your queries as other responses state you could be doing:

     const Test = sequelize.define('test', {
                // attributes
                name: {
                    type: DataType.STRING,
                    allowNull: false
                },
                createdAt: {
                    type: DataType.DATE,
     //note here this is the guy that you are looking for                   
                  get() {
                        return moment(this.getDataValue('createdAt')).format('DD/MM/YYYY h:mm:ss');
                    }
                },
                updatedAt: {
                    type: DataType.DATE,
                    get() {
                        return moment(this.getDataValue('updatedAt')).format('DD/MM/YYYY h:mm:ss');
                    }
                }

If you are using dates to provide info as: last updated, first login, last login. This is the way to go.

The function get returns the new formatted element so it should not be restricted to dates either! Just bear in mind that this will slow your query so use cautiously. ;)

more info (i know i know but docs is the name of the game): http://docs.sequelizejs.com/manual/models-definition.html#getters--amp--setters

you can define custom instance methods getDate/setDate which would translate date between sequelize internal representation and desired format like so http://sequelize.readthedocs.org/en/latest/docs/models-definition/index.html (see 'Expansion of models' section)