This article deals with undated and wrongly dated correspondence of Sergei Bortkiewicz. It presents the hypothetical dates of some letters and postcards as the result of previous studies by other scholars. Other incorrect dates have been corrected using postage stamps and the contents of letters. A description of every undated letter allows us to place them among the others. The article discusses controversial hypotheses, and refutes some of them. Several new hypotheses are put forward instead. Dates for three undated letters by Bortkiewicz are suggested for the first time.