Is the due date for paying Zakat on money the first of Muharram (the beginning of the Hijri year), or at the end of the Hawl (lunar year) for the amount that reaches the Nisab (minimum taxable amount)?

Zakat (definition) becomes obligatory on wealth after the passage of a full Hijri year, not at the beginning of the Hijri year. Rather, it becomes due on the day the complete lunar year has passed since the wealth reached the full nisab (definition) (minimum taxable amount).