Converting Date and Time Values

SPQuery query = new SPQuery();

query.Query = 
    String.Format("<Where><Gt><FieldRef Name='Modified'/>" +
    "<Value Type='DateTime' 
    StorageTZ='TRUE'>{0}</Value></Gt></Where>",
    SPUtility.CreateISO8601DateTimeFromSystemDateTime
    (DateTime.UtcNow.AddDays(-5)));