How to format Date to JSON in golang

March 31, 2021

I have a gorm model and it has a Date field. But golang doesn’t have a “pure” date struct, so we use time.Time.

type OperationPlanningResult struct {
	ID               uint
	PlannedStartedAt time.Time `json:"planned_started_at"`
}

When I marshal it to JSON, this field is like “2006-01-02T00:00:00Z08” and I need to parse it again in frontend.

One way to do it is writing a custom type and providing a custom MarshalJSON function. But I need to write the relative method for gorm because I’m using gorm. Another way is to override the MarshalJSON for the struct.

// FormatDate formats a date. If the t is null, return "null"
func FormatDate(t *time.Time) string {
	if t != nil && !t.IsZero() {
		return t.Format("2006-01-02")
	}
	return "null"
}

// MarshalJSON marshal OperationPlanningResult to json.
func (result OperationPlanningResult) MarshalJSON() ([]byte, error) {
	type Alias OperationPlanningResult
	temp := struct {
		Alias
		PlannedStartedAt string `json:"planned_started_at"`
	}{
		Alias:            (Alias)(result),
		PlannedStartedAt: FormatDate(result.PlannedStartedAt),
	}
	return json.Marshal(temp)
}

However, if you have a lot of gorm models to use the date field, a custom datatype is preferred.