Invalid Dates are being accepted in the Due Date field

Summary

Due date accepts incompatible dates to the configured Date/Time format if manually edited.

Environment

(Optional - If Applicable)

Tested on JIRA 6.4.11

Steps to Reproduce

1. Create a ticket with the default date format
2. Fill the Due Date with the date picker
3. Change the Day/Month/Year format to "dd/MMM/yyyy"
4. Go back to the issue and edit the Date in the Due Date field to something like: 01/01/20016

Expected Results

JIRA would show a message like when an invalid type of date is included in a field (this already happens when you try to include a month as number instead of name): You did not enter a valid date. Please enter the date in the format "d/MMM/yy", eg. "16/Oct/15".

Actual Results

JIRA allows the change to be made. This will make filters stop searching for that issue if based on the due date field, since the date is messed up.

Workaround

Use the default model or just restrict the year back to "yy" in the Day/Month/Year format field.