The method DateTimeFormatterBuilder.appendInstant(int) can throw an exception that is not properly specified. The current spec implies an exception by defining the accepted values as follows: "Specifying zero will cause no fractional digits to be output. From 1 to 9 will output an increasing number of digits, using zero right-padding if necessary. The special value -1 is used to output as many digits as necessary to avoid any trailing zeroes.", however there is no @throws clause. Use of IllegalArgumentException is implied from other similar methods on the class.

Fix proposed as follows:

     * @param fractionalDigits the number of fractional second digits to format with,
     * from 0 to 9, or -1 to use as many digits as necessary
     * @return this, for chaining, not null
     * @throws IllegalArgumentException if the number of fractional digits is invalid
     */
    public DateTimeFormatterBuilder appendInstant(int fractionalDigits) {

(around line 854)

While fixing this issue, the leap second time should be fixed in the Javadoc of the same method. It is currently "23:59:59" but it should be "23:59:60". (The leap second time is a known fact, thus the Javadoc is simply wrong, and this is not a spec change).