In the Qajar era, modern administrative and fiscal demands within the state bureaucracy, alongside encounters with modern sciences, necessitated a critical reassessment of traditional computational methods and calendar systems. Adopting a historical-epistemological approach and drawing upon a direct examination of the lithographic works of Mirza Nezam al-Din Ghaffari (Mohandes al-Mamalek)—most notably Differential and Integral Calculus (Hesab-e Tafazoli va Takamoli) and Analytical Geometry (Hendeseh-ye Tahlili)—this study explicates his pivotal role in introducing modern mathematical analysis and establishing the computational infrastructure for the solar calendar. The central inquiry concerns how Ghaffari, transcending the role of a mere translator, indigenized mathematical concepts and scientific terminology, and how this body of knowledge facilitated the transition from Ptolemaic models to precise calendrical computations. The findings reveal that by coining scientific terminology in Persian and employing differential calculus alongside series expansions, Ghaffari laid a novel foundation for numerical approximation and the calculation of solar motion. This computational shift, which supplanted traditional astronomical tables (zijes) with the principles of celestial mechanics, enabled the precise determination of the vernal equinox and the true length of the solar year, thereby resolving the calendrical discrepancies that had long plagued Qajar bureaucracy. These achievements not only contributed significantly to the formation of modern scientific language in Iran, but also paved the way for the institutionalization and legal formalization of the solar Hijri calendar during the Constitutional era. Consequently, Ghaffari must be recognized as a pioneering agent who bridged advanced mathematics with the organization of national time and the sovereign imperatives of Qajar Iran.