Time always 0 in a go program - json

Question

I am trying here to get trades from Bitfinex API and print them on the screen. Everything works fine, except for times.
I wonder why they always prints as 0001-01-01 00:00:00 in this example...
In UnmarshalJSON, "t" contains the time I need. It just doesn't seem to stick to the Trade structure.
package main
import (
"encoding/json"
"fmt"
"io/ioutil"
"net/http"
"strconv"
"time"
)
type Trade struct {
Tid int
Timestamp myTime
}
type myTime time.Time
func (t myTime) String() string {
when := time.Time(t)
return when.Format("2006-01-02 15:04:05")
}
func (t myTime) UnmarshalJSON(b []byte) error {
ts, err := strconv.Atoi(string(b))
if err != nil {
return err
}
t = myTime(time.Unix(int64(ts), 0))
return nil
}
func main() {
trades, _ := GetTrades()
for _, trade := range trades {
fmt.Printf("%d - %s\n", trade.Tid, trade.Timestamp)
}
}
func GetTrades() (trades []Trade, err error) {
r, err := http.Get("https://api.bitfinex.com/v1/trades/BTCUSD")
defer r.Body.Close()
body, _ := ioutil.ReadAll(r.Body)
json.Unmarshal(body, &trades)
return
}

Answer 1

You can unmarshal on a pointer receiver .. otherwise you're just copying unrelated data around:
func (t *myTime) UnmarshalJSON(b []byte) error {
// ^^^^^^^ this
ts, err := strconv.Atoi(string(b))
if err != nil {
return err
}
*t = myTime(time.Unix(int64(ts), 0))
// ^^ this too
return nil
}
I'm not sure how "safe"/"idiomatic" this is to re-assign data in a pointer receiver.. can someone clarify?
(this works btw.. not sure if its "best practice")

Answer 2

You can't assign in UnmarshalJSON to a value, you have to use a pointer or a struct, one approach is:
type MyTime struct {
time.Time
}
func (t MyTime) String() string {
when := time.Time(t.Time)
return when.Format("2006-01-02 15:04:05")
}
func (t *MyTime) UnmarshalJSON(b []byte) error {
ts, err := strconv.Atoi(string(b))
if err != nil {
return err
}
t.Time = time.Unix(int64(ts), 0)
return nil
}