Tax years for self assessment (sole trader) and corporation tax (Limited Company) are different.
The Self Assessment Tax Year
A tax year for self assessment runs from 6th April each year to the following 5th April. Income tax rates and allowances are set by reference to tax year.
Limited Company Corporation Tax
A tax year for a company runs from 1st April to 31st March with the corporation tax rate set by the company tax year.
This means that for a company it could be the case that the accounting year end is different to the tax year.
This can sometimes mean that different rates of corporation tax apply for the profits made by the business.
Tax software will be used to apportion the profits to tax rates and calculate the corporation tax due.