Validando datas com o go-playground/validator

package main

import (
	"fmt"
	"regexp"

	"gopkg.in/go-playground/validator.v10"
)

func IsISO8601Date(fl validator.FieldLevel) bool {
	regex := "^(-?(?:[1-9][0-9]*)?[0-9]{4})-(1[0-2]|0[1-9])-(3[01]|0[1-9]|[12][0-9])((?:T|\\s)(2[0-3]|[01][0-9]):([0-5][0-9]):([0-5][0-9])?(Z)?)?$"
	ISO8601DateRegex := regexp.MustCompile(regex)

	return ISO8601DateRegex.MatchString(fl.Field().String())
}

type Dates struct {
	ISO8601DateTime string `validate:"ISO8601date"`
	ISO8601Date     string `validate:"ISO8601date"`
	Invalid         string `validate:"ISO8601date"`
}

func main() {
	validator := validator.New()
	validator.RegisterValidation("ISO8601date", IsISO8601Date)

	dates := Dates{ISO8601DateTime: "2019-12-18T14:19:30Z", ISO8601Date: "2019-12-18", Invalid: "2019-60-12"}

	err := validator.Struct(dates)

	if err != nil {
		fmt.Println(err)
	}
}