package main

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

type UnixTime struct {
	time.Time
}

func (t *UnixTime) UnmarshalJSON(data []byte) error {
	var i int64
	if err := json.Unmarshal(data, &i); err != nil {
		return err
	}
	t.Time = time.Unix(i, 0)
	return nil
}

func main() {
	var o = struct {
		Created UnixTime `json:"t"`
	}{}
	_ = json.Unmarshal([]byte(`{"t": 1606089600}`), &o)
	fmt.Printf("%v\n", o.Created)
}