Dynamically calculate average value no matter what type of values (integer, Timestamp, etc.)

I have a dynamic method GetAverage() using reflection to calculate an average value for a list of string values. The string values are dynamically converted to a type determined in ValueType. The type DataSet contains a List<string>:

public class AggregateController {     

    private List<DataSet> DataSets { get; set; }

    private Type ValueType { get; set; }

    public string GetAverage () {
        dynamic sum = Activator.CreateInstance(this.ValueType);

        this.DataSets.ForEach(x => {
            var mI = this.TransformInputValue(x[0]);
            return (dynamic)mI.Invoke(this, new object[] { value });
        });

        return sum.Average().ToString();              
    }

    private dynamic TransformInputValue (string value) {
        try {
            var mI = this.ValueType.GetMethod("Parse", new Type[] { typeof(string) });
            return (dynamic)mI.Invoke(this, new object[] { value });
        } catch {
            return null;
        }
    }
}

This method does everything perfectly when the given values are numeric values. Now I want to achieve dynamically calculating average values for timestamp values with the same method. The problem is that there is no Average() extension method for TimeStamp objects and I do not mean to additionally implement a condition checking for timestamp values like this:

if (this.ValueType == typeof(TimeStamp)) {
    //do stuff to calculate TimeStamp average.
} else {
    return sum.Average().ToString();
}

Is there a way to dynamically react to types like TimeStamp which have no Average() extension method?