The midway of a year is not taken into consideration, but the zakat due date is considered. For example, you Zakat due date is the first of Ramazan , and before the first of Ramazan, your wealth decreased to below the Nisab, but on the coming of the first of Ramazan, your wealth met the Nisab criteria, then Zakat will be due. The drop earlier in the year beneath the Nisab criteria will not be considered