When creating an ad group with version 5.1 of the adCenter API, you can specify that the ad group has no end date by passing null for the EndDate element, but if you pass null for the EndDate element when calling the UpdateAdGroups service operation, the existing EndDate value is not modified. So how do you modify an existing ad group to have no end date?
A change was recently made to adCenter API version 5.1 to support updating an ad group to have no end date. When calling the UpdateAdGroups service operation, simply set the EndDate element to a date that is equal to or later than January 2nd, 2050. adCenter will interpret this as specifying no end date for the ad group and take the appropriate action.
P.S. Don't forget that adCenter API version 4 will no longer be available after August 14th, 2008. You should be migrating your adCenter code to version 5.1 to avoid any interruptions of service.