If you want to use async validation you need to use a runtime that supports Promises.
You can tell to use any A compatible promise implemention like this: There are already many validation libraries out there today but most of them are very tightly coupled to a language or framework.
You don't need to do that when using HTML 5 type="date" attribute (the format then will be forced to be YYYY-MM-DD).
If not, the input is compared to the regular expression.
The expressions use a pre-defined class is returned which enables the form to be submitted.
The goal of is to provide a cross framework and cross language way of validating data.
When setting options via HTML attributes, remember to enable the validator by setting data-fv-date="true".
If the form need to validate start and end date, you should take a look at the Validating fields that depend on each other or Compraing dates example.