Zakah is one of the pillars of Islam. Zakah stands as the third pillar of Islam after testifying that there is no god but Allah and Muhammad is His final Messenger, and offering Prayer. The Prophet (peace and blessings be upon him) said, “Islam is built upon five pillars: testifying that there is no true god except Allah and that Muhammad is the Messenger of Allah, performing Prayer, paying the zakah, making the pilgrimage to the Sacred House (Hajj), and fasting the month of Ramadan” (Reported by Al-Bukhari).
As for calculating zakah on the solar calendar, it is permissible provided that you adjust the rate of zakah to make up for the longer year.
Dr. Monzer Kahf, a prominent economist and counselor, states: The calculation and payment of zakah is supposed to be convenient to the payer; that is why the Prophet (peace and blessings be upon him) ordered the zakah on livestock be collected at a convenient time for their owner, for example, when the livestock is taken for pasture and watering holes. Accordingly, the International Organization of the Contemporary Fiqh of Zakah studied the issue of the lunar calendar and resolved that since many Muslim companies and individuals prepare their balance sheets on a solar calendar, it is permissible to calculate the zakah on a solar calendar provided that the adjustment of the difference in the number of days is taken up to the zakah rates.
Consequently, when you calculate the zakah according to the solar calendar, you need to add 11/354 to the zakah rates, which that will then become 2.577 percent. This is in order to avoid the cost and difficulty of putting another balance sheet and closing accounts.