The Accounting Reference Date or ARD is the date to which annual accounts are made up for a Limited Company.
For example if a company’s ARD is on the 31st of March 2106, accounts must reflect the period between 1st April 2015 to 31st March 2016. The company will then typically have 9 months to file the accounts to Companies House and a further 3 months to file the CT600 tax return.