package main

import (
    "encoding/json"
    "fmt"
    "os"
    "strconv"
    "time"
)

const input = `{"time": "946684799"}`

type Delivery struct {
    Time time.Time `json:"time"`
}

func (d *Delivery) UnmarshalJSON(data []byte) error {
    type Alias Delivery
    aux := &struct {
        Time string `json:"time"`
        *Alias
    }{
        Alias: (*Alias)(d),
    }
    if err := json.Unmarshal(data, &aux); err != nil {
        return err
    }
    i, err := strconv.ParseInt(aux.Time, 10, 64)
    if err != nil {
        return err
    }
    d.Time = time.Unix(i, 0)
    return nil
}

func main() {
    var delivery Delivery
    err := json.Unmarshal([]byte(input), &delivery)
    if err != nil {
        fmt.Println(err.Error())
        os.Exit(1)
    }
    fmt.Println(delivery.Time)
}