Only allowing one year to be selected as "Current" on a Calendar table

I have created my first Access database and have had a lot of success so far with the relationships, form creation including entry rules (using some "light" VBA), and other elements. The one piece which I have not been able to work out is how to only allow one record on my Calendar table to have the "Current" checkbox checked. I have been able to restrict the selection of multiple boxes on the same record, but not over multiple records.

Year | startDate | endDate | Current

2013 | 01/01/13 | 12/31/13 |
2014 | 01/01/14 | 12/31/14 | ×

At this stage, all of the records could be marked as "Current". I would like this to be limited so I can then use this field to display current records when running reports and queries. (In an attempt to help out my end user who will likely be somewhat daunted by too much information.)

Any suggestions are appreciated. Thank you!