Custom validator to compare two fields

I have two fields - StartTime and EndTime - which are both of the type time.Time and they are both within the same struct. How would one go about create a custom validator to ensure that EndTime is within 24 hours of StartTime? I am having trouble setting up the validator.Func to get both field values so that I may compare them (which I know how to do).

var validEndTime validator.Func = func(fl validator.FieldLevel) bool {
    endTime, ok := fl.F().Interface().(time.Time)

	if ok {
		today := time.Now()
		if today.After(date) {
			return false
		}
	}
	return true
}

This is the example function I have started to write.

you can use fl.Parent() to get the struct

package main
import (
	"net/http"
	"time"

	"github.com/gin-gonic/gin"
	"github.com/gin-gonic/gin/binding"
	"github.com/go-playground/validator/v10"
)

// Booking contains binded and validated data.
type Booking struct {
	CheckIn  time.Time `form:"check_in" binding:"required,bookabledate" time_format:"2006-01-02"`
	CheckOut time.Time `form:"check_out" binding:"required" time_format:"2006-01-02"`
}

var bookableDate validator.Func = func(fl validator.FieldLevel) bool {
	date, ok := fl.Field().Interface().(time.Time)
	if ok {
		booking, ok2 := fl.Parent().Interface().(*Booking)
		if ok2 && booking.CheckOut.Unix() - date.Unix() > 24*3600 {
			return false
		}
	}
	return true
}

func main() {
	route := gin.Default()

	if v, ok := binding.Validator.Engine().(*validator.Validate); ok {
		v.RegisterValidation("bookabledate", bookableDate)
	}

	route.GET("/bookable", getBookable)
	route.Run(":8085")
}

func getBookable(c *gin.Context) {
	var b Booking
	if err := c.ShouldBindWith(&b, binding.Query); err == nil {
		c.JSON(http.StatusOK, gin.H{"message": "Booking dates are valid!"})
	} else {
		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
	}
}