Is it permissible for merchants in Yemen to conduct their annual inventory at the end of the Gregorian year, or must it be at the end of the Hijri year, and what is the ruling concerning past years if the Hijri year was indeed obligatory?

Zakat (definition) must be calculated using the Hijri calendar, as it is the calendar prescribed by Islamic law for determining times. It is not permissible to calculate it using the Gregorian calendar except when there is a legitimate need, while ensuring that the obligation (of zakat) is tied to the completion of the Hijri year (hawl), and that the difference resulting from the delay must be accounted for, which amounts to the zakat of one lunar year for every 33 solar years completed.