"code": "DateFormatError",
	"target": "Unknown",
	"details": [
			"code": "DateFormatError",
			"number": 26,
			"message": "Dates are only recognized in ISO 8601 format.",
			"description": "The field '-0-' was not recognized as a valid date value.  Valid dates are in the ISO 8601 format.",
			"faultCode": "Client",
			"helpLink": "",
			"severity": "Error"

Th is indicates that the date value you provided was incorrectly formatted. AvaTax only supports date values in ISO 8601 format. The ISO 8601 format has two variants; one is for date-only and one is for date-plus-time.

A standard ISO 8601 date-only value is in Year-Month-Day format, for example, 2017-03-01. It must always have a four digit year, a two digit month, and a two digit day, each separated by hyphens.

An ISO date-plus-time value is in Year-Month-Day followed by Hour:Minute:Second, for example, 2017-03-31T23:04:16.