Hi, Keith, Both CreateDate and UpdateDate are regular non-dynamic properties of DynamicNode. Since that they are typed as DateTime and you cannot pass them to the DateDiff method because it expects strings. If you much want to use the very DateDiff you can do it getting {Create/Update}Date-s converted to strings:

var diff = umbraco.library.DateDiff(Model.CreateDate.ToString("s"), Model.UpdateDate.ToString("s"), "m");

However you absolutely don't need it - since these props are DateTime-typed and you can write simply:

var diff = Model.UpdateDate - Model.CreateDate;