When converting from one system to another, a one figure is added or subracted to the resulting figure.
For reference one can use the table below.
In the Qur'aan a reference to the octal system is found by taking the verse and Surah index of Ayah 4, Surah Al-Maariij and in octals mode converting it to octals removing the colon (:) as-is. Then we convert to decimals (1988).
Then, we add .7619 to the figure - a valid reference to a 7619 day katun. And multiply by 21 to obtain 41760.9999 - comparable to the 41760 equivalent in a zero-based octal system and at the same time approximate to the figure equivalent in an one-based octal system used, Allah knows best, in old times.
Surah Al-Qasas makes a vague reference to the octal system (28:27) but not enough to justify its application to Islamic culture. Please keep on reading to learn more.