How is the beginning of the Hawl (Zakat year) calculated for Zakat on dollar savings, and is the valuation done at the official dollar exchange rate, or in the parallel market, or at the global price per gram of gold?

The obligation of Zakat (definition) begins when savings reach the nisab (definition), which is equivalent to 85 grams of 24-carat gold. Zakat is calculated after a full lunar year (hawl) has passed since the wealth reached the nisab. It is obligatory to give out one-quarter of one-tenth (2.5%) of all savings upon which a full year has passed (by dividing the amount by 40). It is permissible to pay Zakat in cash in dollars or its equivalent in local currency. It is preferable to use the higher exchange rate when paying in local currency to serve the best interest of the poor.