code: func GetFracIndex(s string) (index int) {
  	index = -1
  	for i := len(s) - 1; i >= 0; i-- {
  		if unicode.IsPunct(rune(s[i])) {
  			if s[i] == '.' {
  				index = i
  			}
  			break
  		}
  	}
  
  	return index
  }
docs: // GetFracIndex finds the last '.' for get fracStr, index = -1 means fracStr not found.
  // but for format like '2019.01.01 00:00:00', the index should be -1.