string isoDate should allow the string to be converted to same format as date iso schema

[bdgamble]

Context

• node version: all
• joi version: 10.5.0
• environment (node, browser): node
• used with (hapi, standalone, ...): standalone
• any other relevant information:

What are you trying to achieve or the steps to reproduce ?

Due to a limitation with using joi as the schema for DynamoDb via https://github.com/ryanfitz/vogels I need to validate dates with joi.string().isoDate() and use the value returned from validation to store in DynamoDb. I need the date to be formatted in the simplified extended ISO format in the same way that joi.date().iso() would return.

const stringSchema = Joi.string().isoDate();
const dateSchema = Joi.date().iso();

const stringResults = Joi.validate('2017-01-01', stringSchema);
const dateResults = Joi.validate('2017-01-01', dateSchema);

// want these to be the same
console.log('string value: ', stringResults.result);
console.log('date value: ', dateResults.result);

Which result you had ?

The string schema just returns the provided string. i.e "2017-01-01" returns "2017-01-01"

What did you expect ?

I want it to return "2017-01-01T00:00:00.000Z", or an option to convert it as such