Some strings get converted into unexpected dates in some cases

Steps to reproduce

• Create an XWiki class with a date property having format "MM/yyyy"
• In the BatchImport configuration, specify the default date format as "dd/MM/yyyy"
• Import data containing dates with format "dd/MM/yyyy", for example a row containing the string value "01/03/2019"

Expected behaviour

The data gets imported with dates resulting from a parsing using the default format, since the data format is not the one declared in the class. For example "01/03/2019" should be interpreted as March 1 00:00:00 2019.

Actual behaviour

The data gets imported with wrong dates. For example "01/03/2019" is interpreted as Mon Jan 01 00:00:00 CET 3.

Notes towards a fix

Java example of the issue

String dateAsString = "01/03/2019";
String format = "MM/yyyy";
SimpleDateFormat formatter = new SimpleDateFormat(format);
format.setLenient(false); // does actually not change the output
Date date = formatter.parse(dateAsString);
System.out.println(date);

Produces the output below, while a ParseException would be more expected:

Mon Jan 01 00:00:00 CET 3

Possible solutions

Check that the parsed date, once re-formatted as a string, is equal to the original string

if (!dateAsString.equals(formatter.format(date))) { 
  throw new ParseException("Invalid date format", 0);
}

Reference: https://rolfje.wordpress.com/2010/03/06/simple-strict-date-parsing/

Use DateTimeFormatter

DateTimeFormatter provides a more strict parsing by default, and advanced strictness configuration levels. With the code below, an attempt to parse "01/03/2019" will throw an exception, as expected. The wiki or user timezone should to be used instead of the system default.

DateTimeFormatter formatter = new DateTimeFormatterBuilder().appendPattern(format)
 .parseDefaulting(ChronoField.YEAR_OF_ERA, ZonedDateTime.now().getYear()) .parseDefaulting(ChronoField.DAY_OF_MONTH, 1)
 .parseDefaulting(ChronoField.DAY_OF_MONTH, 1)
 .parseDefaulting(ChronoField.HOUR_OF_DAY, 0)
 .parseDefaulting(ChronoField.MINUTE_OF_HOUR, 0)
 .parseDefaulting(ChronoField.SECOND_OF_MINUTE, 0) .toFormatter(); LocalDate dateTime = LocalDate.parse(dateAsString, formatter); System.out.println(localDate); date = Date.from(localDate.atStartOfDay().atZone(ZoneId.systemDefault()).toInstant());