In order for zakaah to be obligatory, one year must have passed, i.e., one hijri year must have gone by since the wealth reached the nisaab (threshold at which zakaah becomes due). The Prophet (peace and blessings of Allaah be upon him) said: “There is no zakaah on wealth until a year has passed.” (Narrated by Ibn Maajah; classed as saheeh by al-Albaani in Irwa’ al-Ghaleel, no. 787).
The passage of one year is also a condition for zakaah on gold, silver, paper currency and animals of the an’aam class to be obligatory. [Translator’s note: the an’aam class of animals includes camels, cattle, sheep and goats].
Wealth that is earned during the year is of two types:
1 – That which is profit generated by other wealth. Zakaah on these profits is to be paid at the same time as zakaah on the principle is due.
2 – That which is independent wealth which is gained in a manner acceptable according to sharee’ah, such as inheritance, gifts and savings from one's salary. No zakaah is due on this until one year has passed from the date when it reached the nisaab.
The Muslim may pay the zakaah on all the wealth he has in Ramadaan, both for that for which one year has passed and for that for which one year has not passed, in order to expedite the zakaah, because of the report narrated by al-Tirmidhi, Ibn Maajah and al-Haakim, who classed it as saheeh, that al-‘Abbaas (may Allaah be pleased with him) asked the Messenger of Allaah (peace and blessings of Allaah be upon him) about expediting his zakaah before one year had passed, and he allowed him to do that. This hadeeth was classed as hasan by al-Albaani in Saheeh al-Tirmidhi, no. 545.
This is easier than calculating the zakaah for each part of his wealth independently, so that he will not get confused about his wealth and mix up the figures, which may result in part of the zakaah not being paid or in confusion as to whether the full of amount of zakaah has been paid or not.
See also question no. 26113
And Allaah knows best.