When operationSpec for an API response specifies an object property with format "Date" its value is not deserialized to Javascript Date object as expected.

Swagger / Open API example:

"dateOfIssue": {
          "format": "date",
          "description": "Date of issue",
          "type": "string"
        },

For example when value "2021-03-31" is supplied by API in aforementioned property dateOfIssue the resulting Date object is actually deserialized as "2021-03-30" because the local timezone offset is applied.

Code in question is:

This is due to a rather strange behaviour of Javascript Date object when local time is set to negative UTC offset and only a date value is passed to its contructor:

// Timezone set to -6
new Date("2021-03-31")  // is in fact interpreted as "Tue Mar 30 2021 18:00:00 GMT-0600 (Central Standard Time)"

The above issue does not apply to positive UTC offsets.

However, when time fraction is supplied (with midnight) it yields expected results:

// Timezone set to -6
// when provided with time fraction (and without explicit time zone) it is constructed properly
new Date("2021-03-31T00:00:00")  // is in fact interpreted as "Wed Mar 31 2021 00:00:00 GMT-0600 (Central Standard Time)"

I believe that Date only values should be interpreted strictly as local date regardless of currently used time zone offset. The following fix could be used to provide such a behaviour:

else if (mapperType.match(/^(Date)$/ig) !== null) {
      payload = new Date(responseBody + "T00:00:00");
}
else if (mapperType.match(/^(DateTime|DateTimeRfc1123)$/ig) !== null) {
      payload = new Date(responseBody);
}

The very same behaviour is actually also present in the newer version of azure-sdk-for-js.