Praise be to Allah.
If someone has gold that reaches the minimum threshold at which zakaah becomes due (nisaab), then he buys new gold during the year, or receives it as a gift and the like, then he should start a new year for the newly-acquired gold, from the day he bought it or accepted it as a gift.
Based on that, he should give zakaah on the old gold when one year has passed, then give zakaah on the new gold when its year has passed, starting with the date it came into his possession.
If he wishes, he may give zakaah on all the gold when one year has passed for the old gold, in which case he will have given zakaah on the new gold before its year has passed. This is permissible, and it is called giving zakaah in advance.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was asked about a case where someone buys new gold and adds it to old gold on which he has to give zakaah.
He replied: If he buys gold during the year, then it is not to be added to any previous gold with regard to zakaah. Rather he should make a separate year for it. But if he wants to add it to the previous gold and give zakaah on both at the same time, this is fine and it comes under the heading of giving zakaah ahead of time.
Question: What if the new gold is below the nisaab (minimum threshold)?
Shaykh: If what he bought is below the minimum threshold, it should be added to the old gold in order to determine whether it reaches the minimum threshold, but with regard to the time at which zakaah becomes due, he may separate them, unless he chooses to give zakaah on both at the same time.
End quote from Liqa’ al-Baab al-Maftooh.
And Allah knows best.