MyError.go

package main

import (
	"fmt"
	"time"
)

// Anything that has Error() function is an Error, i.e. satisfied the Error
// interface.

type MyError struct {
	what string
	when time.Time
}

func (m MyError) Error() string {
	return fmt.Sprintf("%s at %s", m.what, m.when)
}

func main() {
	err := MyError{}
	err.what = "Bad thing"
	err.when = time.Now()
	fmt.Printf("%v\n", err)
}