code: func parseTimestamp(s string) (time.Time, bool) {
  	// TODO write code to check all the formats supported by
  	// http://yaml.org/type/timestamp.html instead of using time.Parse.
  
  	// Quick check: all date formats start with YYYY-.
  	i := 0
  	for ; i < len(s); i++ {
  		if c := s[i]; c < '0' || c > '9' {
  			break
  		}
  	}
  	if i != 4 || i == len(s) || s[i] != '-' {
  		return time.Time{}, false
  	}
  	for _, format := range allowedTimestampFormats {
  		if t, err := time.Parse(format, s); err == nil {
  			return t, true
  		}
  	}
  	return time.Time{}, false
  }
docs: // parseTimestamp parses s as a timestamp string and
  // returns the timestamp and reports whether it succeeded.
  // Timestamp formats are defined at http://yaml.org/type/timestamp.html