Add some validator and warning message in UI when a user enters only 2 digits of the year in format dd/MM/yyyy

Problem

Add some validator and warning message in JIRA when a user enters a date (year part) using dd/MM/yy, while the input date format is set to dd/MM/yyyy

Steps to Reproduce:

1. Go to Administration > General Configuration > Advanced.
2. Change the jira.date.picker.java.format from d/MMM/yy to dd/MM/yyyy.
3. Create or edit an issue and type a number date in the Due Date field with format d/m/yy such as 1/1/14 for example
4. Then click create or edit.
5. After it is created or updated, go to the issue and click edit.

Actual Results:

The date typed is accepted and the Due Date field is storing 01/01/0014 instead of 2014.

Suggestion

Add some validator and warning message in JIRA UI when a user enters a date using dd/MM/yy, while the input date format is set to dd/MM/yyyy, so that the wrong year does not get set in JIRA.

Notes

It was decided in this original bug ticket that this behavior was actually not a bug, but an expected behavior due to the following reason:

For parsing, if the number of pattern letters is more than 2, the year is interpreted literally, regardless of the number of digits. So using the pattern "MM/dd/yyyy", "01/11/12" parses to Jan 11, 12 A.D.

source:

• http://docs.oracle.com/javase/6/docs/api/index.html?java/text/SimpleDateFormat.html
• https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/text/SimpleDateFormat.html

Having said that, in order to avoid any confusion for JIRA users, it would be great to do a check on JIRA side on the input data before the date gets processed in the backend by the Java class.