Zakat is payable on the last day of the anniversary of owning Nisab (354th day). So, if that happens on 20th Rajab, your next Zakat will be due 20th Rajab of the following year.
You may change your Zakat due date, to make it in Ramadan or to make it consistent with your business and accounting cycle. This is done by calculating the number of days difference and pay their Zakah separately.