code: func (c Calendar) Format(datetime time.Time, pattern string) (string, error) {
  	parsed, err := c.parseDateTimeFormat(pattern)
  	if err != nil {
  		return "", err
  	}
  
  	return c.formatDateTime(datetime, parsed)
  }
docs: // Format takes a time struct and a format and returns a formatted
  // string. Callers should use a DateFormat, TimeFormat, or DateTimeFormat
  // constant.