You must save the standard .NET DateTime object in SPFieldDateTime, for example:
using (SPSite site = new SPSite("http://YOUR URL")) { using (SPWeb web = site.OpenWeb()) { SPList list = web.Lists["news"]; SPListItem item = list.Items.Add(); DateTime dt = DateTime.Now; item["Title"] = "Test"; item["Expires"] = dt; item.Update(); } }
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