in ecs-logging-core/src/main/java/co/elastic/logging/TimestampSerializer.java [98:105]
private void serializeWithLeadingZero(StringBuilder builder, long value, int minLength) {
for (int i = minLength - 1; i > 0; i--) {
if (value < Math.pow(10, i)) {
builder.append(ZERO);
}
}
builder.append(value);
}