Array Sorting Headaches

Greetings everyone : Having a hard time with this one. I have a multi-dim array $foo[$x][$y]['key'], where $x and $y are numeric. Here is some sample data : Array ( [$x] => Array

        (
            [0] => Array
                (
                    [invoiceid] => 11842
                    [product] => myproduct
                    [domain] => foo.net
                    [expires] => February 28, 2004
                )

            [1] => Array
                (
                    [invoiceid] => 10295
                    [product] => myotherproduct
                    [domain] => foo.net
                    [expires] => December 9, 2003
                )

            [2] => Array
                (
                    [invoiceid] => 10202
                    [product] => product1
                    [domain] => foo.bar
                    [expires] => January 19, 2003
                )

            [3] => Array
                (
                    [invoiceid] => 10005
                    [product] => product2
                    [domain] => foo.bar
                    [expires] => December 11, 2002
                )

        )

) I need to filter the results so that I get the latest expiry date for each product. The expires field actually contains a timestamp. So for the sample array above, the resultant array would have only keys 0 and 2, filtering out all the rest. There are around 180+ main entries, with any number of sub entries, but each sub entry has the same four keys. Any ideas? Getting a rather large headache mulling over this. Thanks again, Burhan