The article describes chronological characteristics of the written monuments in «Сlear Script». The development process of the peculiar Oirat (Kalmyk) literature, written in «Clear Script» can be divided into two periods. The 1st period is from 1648 to 1771. The second period is divided into two parts which is connected with the fact that the major part of the Kalmyk people left for Dzhungaria in 1771 and the unified Oirat-Kalmyk written language continued to evolve in different geographical and ethno-political conditions. On the territory of the Lower Volga region the old Kalmyk written language began to develop, the graphic system of which was replaced in 1924 by the Cyrillic alphabet. A conventionally «Oirat» written language has been further developed in Xinjiang which continues to be used at the present time. While studying the chronological characteristics of written heritage in «Clear Script» original works of the authors of the XVII-XIX centuries, primarily historical essays, formal written documentation, as well as works created on the basis of works of folklore will be selected as text sources for the subcorpus of «early texts». These works convey a vivid world of the Oirat and the Kalmyk people’s way of life in the XVII-XIX centuries. Handwritten historical essays, business letters, namtaru and other works of original genres, despite their brevity, are of special cultural value, as they reflect, albeit in a limited way, the state of the Oirat-Kalmyk language of the time and carry information about the grammatical system of the Kalmyk language.