Golangのgormで、カスタマイズしたtime.Timeに差し変える ref: http://qiita.com/roothybrid7/items/52623bedb45ff0c26a8a

file0.go

// gorm.Model
type Model struct {
    ID        uint64 `gorm:"primary_key"`
    CreatedAt time.Time
    UpdatedAt time.Time
    DeletedAt *time.Time
}

file1.go

// Time Format
const (
    RFC3339     = "2006-01-02T15:04:05Z07:00"
    RFC3339Nano = "2006-01-02T15:04:05.999999999Z07:00"
)

// JSON Encode
func (t Time) MarshalJSON() ([]byte, error) {
    if y := t.Year(); y < 0 || y >= 10000 {
        // RFC 3339 is clear that years are 4 digits exactly.
        // See golang.org/issue/4556#c15 for more discussion.
        return nil, errors.New("Time.MarshalJSON: year outside of range [0,9999]")
    }
    return []byte(t.Format(`"` + RFC3339Nano + `"`)), nil
}

file3.go

type Model struct {
    ID        uint64 `gorm:"primary_key"`
    CreatedAt ResourceTime `json:"createdAt"`
    UpdatedAt ResourceTime `json:"updatedAt"`
    DeletedAt *time.Time   `json:"deletedAt,omitempty"`
}

restime.go

package typedef

import (
    "database/sql/driver"
    "errors"
    "time"
)

// XXX: アプリでナノ秒は必要ないので秒まででフォーマット
//{
//    "createdAt": "2015-08-09T15:03:05.135166971Z",
//    "objectID": "55c112a40000a1"
//}
// See: http://qiita.com/taizo/items/2c3a338f1aeea86ce9e2
type ResourceTime struct {
    time.Time
}

func (rt ResourceTime) MarshalJSON() ([]byte, error) {
    t := rt.Time
    if y := t.Year(); y < 0 || y >= 10000 {
        return nil, errors.New("ResourceTime.MarshalJSON: year outside of range [0,9999]")
    }
    return []byte(t.Format(`"` + time.RFC3339 + `"`)), nil
}

func (rt ResourceTime) MarshalText() ([]byte, error) {
    t := rt.Time
    if y := t.Year(); y < 0 || y >= 10000 {
        return nil, errors.New("Time.MarshalText: year outside of range [0,9999]")
    }
    return []byte(t.Format(time.RFC3339)), nil
}

// Sql driver interface

func (rt *ResourceTime) Scan(value interface{}) error {
    rt.Time = value.(time.Time)
    return nil
}

func (rt ResourceTime) Value() (driver.Value, error) {
    return rt.Time, nil
}