Is Zakat obligatory on all owned wealth, or only on the amount that first reached the Nisab?

The determining factor for the month of zakat (definition) is the lunar months, not the Gregorian months, due to the Almighty’s saying: ﴿They ask you about the new moons. Say: They are fixed times for people and for the Hajj.﴾ So, you must consider the Hijri month in which the wealth reached the nisab (definition), and you should give out a quarter of a tenth after a full year has passed from that month, provided that the wealth does not fall below the nisab. As for new wealth, if it is a profit from the original wealth, it is zakatable along with it. Otherwise, zakat is not obligatory on it until a full year has passed over it. It is permissible to combine it with the original wealth and pay zakat on all of it when the year for the original wealth is complete.