Zakat on Savings is based on 2.5% of the lowest savings balance in a haul which is 355 Gregorian calendar days. This period of 355 days is different for each individual and does not have to be from pay day to pay day, Ramadan to Ramadan, or Jan to Dec. If for example your haul is from Feb to Jan the following year, the Zakat will be based on 2.5% of the lowest balance that you have from Feb to Jan. The haul is determined by the nisab value. If in one year, your savings balance does not fall below the nisab value, then you have the haul in existence. If at any point in time your savings balance drops below the nisab, there is no haul. So for some people, they do not have to pay Zakat as their haul is always broken and never completes a full Hijrah year. The nisab value, which is used to determine if there is a haul, is the nisab value at the end of the 355 days period.