CSV Import with date field fails to import when dates are within the last 7 days

Steps to reproduce

1. Start with JIRA 6.4-OD-13.
2. Create a new custom field of type "Date Picker".
3. Create a simple CSV file:

   Summary,Date Picker
   Test Issue 1,5/Feb/15
   Test Issue 2,10/Feb/15
   Test Issue 3,24/Feb/15
   

4. Import that file using the JIRA Importers Plugin's CSV Importer (use "dd/MMM/yy" for the date format), note that the Date Picker field is populated in all three imported issues.
5. Upgrade to JIRA 6.4-OD-14 or 15, repeat this process.

Expected results

All issues are imported and the Date Picker field is correctly populated with the date specified in the CSV.

Actual results

The issues are imported successfully, but on "Test Issue 3", the Date Picker field isn't filled in and JIM returns a message at the end of the import process:

Cannot add value [ Tuesday 12:00 AM ] to CustomField Date Picker in Issue null. Exception Message: Invalid date format. Please enter the date in the format "d/MMM/yy".

That message will show for any date that's within the last 7 days. So if the import is run today (the 2nd of March), it'll occur only for the 24th of February onwards. Come tomorrow, it'll be the 25th, and so on.