The consensus among linguists is that modern, standard Czech originated during the eighteenth century. By then the language had developed a literary tradition, and since then it has changed little; journals from that period have no substantial differences from modern standard Czech, and contemporary Czechs can understand them with little difficulty. Changes include the morphological shift of í to ej and é to í (although é survives for some uses) and the merging of í and the former ejí. Sometime before the eighteenth century, the Czech language abandoned a distinction between phonemic /l/ and /ʎ/ which survives in Slovak.
When do linguists agree modern Czech originated? (If the question is unanswerable, say "unanswerable")
the eighteenth century