Retrieving value from HTML table
If I understand correctly, the role of this procedure (which is far from obvious from the name alone) is to assign either _CreateUTC or _ModifyUTC values (which should be _createUTC and _modifyUTC if they're private fields - always use camelCase for fields and locals/parameters.

You don't need that rows list of lists of strings at all, let alone iterate that monster afterward to retrieve a value.

Just loop and locate the value as you're looping, don't store anything but what you need and stop looping once you got it - I think something like this would do it:

var headerIndex = 0;
DateTime value;
foreach (var htmlRow in htmlRows)
{
    if (htmlRow.FindElements(By.TagName("th")).Count &gt; 0)
    {
        headerIndex = tableHeaders.Select(th =&gt; th.Text.ToString()).IndexOf(columnHeader);
        continue;
    }

    var cellsInRow = htmlRow.FindElements(By.TagName("td")).ToArray();
    if (cellsInRow[0] == entryName)
    {
        value = Convert.ToDateTime(cellsInRow[headerIndex].Text.ToString());
        break;
    }
}

if (columnHeader = "Create UTC Date")
{
    _createUTC = value;
}
else if (columnHeader = "Modify UTC Date")
{
    _modifyUTC = value;
}