It is very close to the Middle Mongol language, the language spoken at the time of Genghis Khan and the Mongol Empire.
An exception would be the Common Mongolic pluritative voice suffix -cAgA- 'do together', which can be reconstructed from the modern languages but is not attested in Middle Mongol.
Regarding the time period when Proto-Mongolic was spoken, Juha Janhunen writes: "The absolute dating of Proto-Mongolic depends on when, exactly, the linguistic unity of its speakers ended", that is, when it evolved into separate Mongolic languages; this event took place "only after the geographical dispersal of the ancient Mongols under Chinggis Khan", which was "not earlier than the thirteenth century."
"[2] One way in which Proto-Mongolic formed plurals was by adding -s or -ud to a word.
-ud would be added to words ending in consonants, for example nom (book) would become nomud.