How to parse date-time with two or three milliseconds digits in java?

You would need to build a formatter with a specified fraction

DateTimeFormatter formatter = new DateTimeFormatterBuilder()
  .appendPattern("yyyy-MM-dd HH:mm:ss")
  .appendFraction(ChronoField.MILLI_OF_SECOND, 2, 3, true) // min 2 max 3
  .toFormatter();

LocalDateTime formatDateTime = LocalDateTime.parse(date, formatter);

tl;dr

No need to define a formatter at all.

LocalDateTime.parse(
    "2017-11-21 18:11:14.05".replace( " " , "T" )
)

ISO 8601

The Answer by Sleiman Jneidi is especially clever and high-tech, but there is a simpler way.

Adjust your input string to comply with ISO 8601 format, the format used by default in the java.time classes. So no need to specify a formatting pattern at all. The default formatter can handle any number of decimal digits between zero (whole seconds) and nine (nanoseconds) for the fractional second.

Your input is nearly compliant. Just replace the SPACE in the middle with aT.

String input = "2017-11-21 18:11:14.05".replace( " " , "T" );
LocalDateTime ldt = LocalDateTime.parse( input );

ldt.toString(): 2017-11-21T18:11:14.050

---

The answers by Basil Bourque and Sleiman Jneidi are excellent. I just wanted to point out that the answer by EMH333 has a point in it too: the following very simple modification of the code in the question solves your problem.

    DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss.[SSS][SS]");

The square bracket in the format pattern string enclose optional parts, so this accepts 3 or 2 decimals in the fraction of seconds.

• Potential advantage over Basil Bourque’s answer: gives better input validation, will object if there is only 1 or there are four decimals on the seconds (whether this is an advantage depends entirely on your situation).
• Advantage over Sleiman Jneidi’s answer: You don’t need the builder.

Possible downside: it accepts no decimals at all (as long as the decimal point is there).

As I said, the other solutions are very good too. Which one you prefer is mostly a matter of taste.