package main

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

type MyStruct struct {
	Name           string    `json:"name"`
	SomeCustomType time.Time `json:"someCustomType"`
}

func (s *MyStruct) UnmarshalJSON(data []byte) error {
	type Alias MyStruct
	aux := &struct {
		SomeCustomType int64 `json:"someCustomType"`
		*Alias
	}{
		Alias: (*Alias)(s),
	}
	if err := json.Unmarshal(data, &aux); err != nil {
		return err
	}
	s.SomeCustomType = time.Unix(aux.SomeCustomType, 0)
	return nil
}

func main() {
	data := []byte(`{"name":"bob", "someCustomType": 152108680}`)
	var myStruct *MyStruct
	err := json.Unmarshal(data, &myStruct)
	if err != nil {
		fmt.Println(err)
	}

	log.Println(myStruct)
}