Preheat the oven to 350°. In a large skillet, heat the 3/4 cup of olive oil with the garlic and cook over moderate heat until the garlic is golden. Using a slotted spoon, remove the garlic and reserve for another use. Add half of the parsnips to the skillet and cook over moderate heat, stirring occasionally, until golden and barely tender, about 12 minutes. Using a slotted spoon, transfer the parsnips to a roasting pan. Repeat with the remaining parsnips, then scrape the parsnips and any oil into the roasting pan. Add the dates and marjoram sprigs, season with salt and pepper and roast for about 8 minutes, just until the parsnips are tender and the dates are slightly caramelized. Transfer the parsnips and dates to a platter.