Ziegler provides a cool intro, implementation sample and much more. When deployed we can apply this policy to a contenttype in the UI, or in code. To create our own expiration logic we have to implement IExpirationFormula and its ComputeExpireDate:

public class TaskExpiration : IExpirationFormula
{
	public DateTime? ComputeExpireDate(SPListItem item,
						XmlNode parametersData)
	{
		if (!item\["Status"\].Equals("Completed"))
		{
			return null;
		}
		var dt = (DateTime) item\["Modified"\];
		return dt.AddDays(30);
	}
}
```In order to see IExpirationFormula, add a reference to Microsoft.Office.Policy (and maybe Microsoft.Office.DocumentManagement): [![](https://sharepointkunskap.files.wordpress.com/2011/12/policy-dll.png "policy-dll")](https://sharepointkunskap.files.wordpress.com/2011/12/policy-dll.png) To see our custom retention policy, we have to register it in xml, we can do it in Feature Receiver like [Yaroslav](http://www.sharemuch.com/2011/01/10/creation-custom-retention-policies-for-sharepoint-2010-libraries/):

public override void FeatureActivated(SPFeatureReceiverProperties properties) {     const string xmlManifest =        “<PolicyResource xmlns=‘urn:schemas-microsoft-com:office:server:policy’” +        " id = ‘Takana.TaskRetentionPolicy’" +        " featureId=‘Microsoft.Office.RecordsManagement.PolicyFeatures.Expiration’" +        " type = ‘DateCalculator’>" +        " Takana Task Retention Policy" +        “Tasks expire 30 days after they have been completed” +        “Takana.SharePoint, Version=1.0.0.0, Culture=neutral, " +        “PublicKeyToken=920c0327f8b01d97” +        “Takana.SharePoint.Policies.TaskExpiration” +        “”;     PolicyResourceCollection.Add(xmlManifest); }