Not really a Kotlin question but SO provides ``` v...

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[nkiesel]

Not really a Kotlin question but SO provides

val formatter = DateTimeFormatterBuilder()
    .appendPattern("yyyy-MM-dd'T'HH:mm:ss")
    .appendFraction(ChronoField.MILLI_OF_SECOND, 1, 3, true) // min 1, max 3 digits
    .toFormatter()
val formatDateTime = LocalDateTime.parse(value, formatter)