a.go

package main

import (
	"errors"
	"fmt"
	"reflect"
	"time"

	"github.com/lestrrat/go-urlenc"
)

type Bar struct {
	T MaybeTime `urlenc:"t,omitempty,string"`
}

type MaybeTime struct {
	Valid bool
	T     time.Time
}

func (m MaybeTime) Value() interface{} {
	return m.T
}

func (m *MaybeTime) Set(v interface{}) error {
	switch v.(type) {
	case string:
		m.Valid = true
		t, err := time.Parse(`2006-01-02`, v.(string))
		if err != nil {
			return err
		}
		m.T = t
	case time.Time:
		m.Valid = true
		m.T = v.(time.Time)
	default:
		return errors.New("expected string (got: " + reflect.TypeOf(v).String() + ")")
	}
	return nil
}

func main() {
	const src = `t=2017-01-01`

	var m Bar
	err := urlenc.Unmarshal([]byte(src), &m)
	if err != nil {
		panic(err)
	}
	fmt.Printf("v = %+v\n", m)
}