Any fluctuation during the year above nisab is not relevant to paying zakat, as long as one maintains nisab. For example, If one earns $5,000 the day before the zakat due date, one pays zakat on the total amount that remains in one’s possession on the zakat due date.
So in the case of a person who owns 35 head of cattle on the zakat due date, who maintains the nisab of 30 or more head of cattle throughout the 12 lunar months in their entirety, zakat is due on all 35 cows.
This is considered the most applicable method and strongest opinion, held by the majority of scholars. Shaykh Yusuf al-Qardawi argues that the requirement of the passage of one year is not necessary on earnings or other zakatable wealth that is acquired and added to nisab (Fiqh az-Zakat, 310, 313, 316, 321-325). In fact, all four Schools of Law concur, provided nisab is maintained.
Likewise, for merchants and businessmen possessing nisab from the first day of establishing their business, their zakat is due exactly one lunar year from that date.