C# - How to query a Microsoft Certification Authority by a date value using the ICertView::SetRestriction COM method?

I am attempting to write a program that can query a Microsoft CA for certificates which were revoked after a given date. I was able to instantiate a CCertView object, however, I'm not sure how to pass the date parameter to the SetRestriction method, which is (poorly) documented here.

The method requires 4 variables:

HRESULT SetRestriction(
   LONG          ColumnIndex,
   LONG          SeekOperator,
   LONG          SortOrder,
   const VARIANT *pvarValue
);

The parameter I'm having trouble with is *pvarValue. I confirmed that I can pass in a reference to an int if I want to filter based on a simple column (such as request ID). However, I need to pass in a date value.

Here's what I have so far:

CCertView cView = new CCertView();

        //Establish the connection to the CA
        cView.OpenConnection(@"certserver.contoso.com\Contoso Issuing CA");

        //Indicate which columns that I want to appear in my results view
        cView.SetResultColumnCount(4);
        cView.SetResultColumn(ADCS_COL_SerialNumber);
        cView.SetResultColumn(ADCS_COL_CommonName);
        cView.SetResultColumn(ADCS_COL_Request_Disposition);
        cView.SetResultColumn(ADCS_COL_Request_RevokedEffectiveWhen);

        //First, I'm going to filter the results for all REVOKED certificates (Disposition=21)
        object dispositionRevoked = 21;

        //Then, I want to filter further to isolate only certs that were revoked on or after 7/20/18
        object filterDate = "7/20/2014";

        //Here I set the first filter (Disposition=21).  This works.
        cView.SetRestriction(ADCS_COL_Request_Disposition, CVR_SEEK_EQ, CVR_SORT_ASCEND, ref dispositionRevoked);

        //This is where I'm attempting to define the date filter.  This does NOT work.
        cView.SetRestriction(ADCS_COL_Request_RevokedWhen, CVR_SEEK_GE, CVR_SORT_ASCEND, ref filterDate);


        IEnumCERTVIEWROW rows = cView.OpenView();
        IEnumCERTVIEWCOLUMN column;
        rows.Reset();

        int index;
        while (rows.Next() != -1)
        {
           ...

When I run the program, I receive the following exception:

C:\Users\admin\Desktop>getRevoked.exe
Unhandled Exception: System.ArgumentException: CCertView::SetRestriction: The
parameter is incorrect. 0x80070057 (WIN32:87)
     at CERTADMINLib.ICertView2.SetRestriction(Int32 ColumnIndex, Int32 SeekOperator, Int32 SortOrder, Object& pvarValue)
     at getRevoked.Program.Main(String[] args)