public class ItemYear : AbstractComputedIndexField
{
    public override object ComputeFieldValue(IIndexable indexable)
    {
        Item item = indexable as SitecoreIndexableItem;

        DateTime date = EXTRACT DATE FROM THE ITEM;

        return date.Year;
    }
}

<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/">
    <sitecore>
        <contentSearch>
            <indexConfigurations>
                <defaultLuceneIndexConfiguration type="Sitecore.ContentSearch.LuceneProvider.LuceneIndexConfiguration, Sitecore.ContentSearch.LuceneProvider">
                    <documentOptions type="Sitecore.ContentSearch.LuceneProvider.LuceneDocumentBuilderOptions, Sitecore.ContentSearch.LuceneProvider">
                        <fields hint="raw:AddComputedIndexField">
                            <field fieldName="year" returnType="int">NAMESPACE.ItemYear , ASSEMBLY_NAME</field>
                        </fields>
                    </documentOptions>
                </defaultLuceneIndexConfiguration>
            </indexConfigurations>
        </contentSearch>
    </sitecore>
</configuration>