The HMRC Basis Period for Self Assessment - Definition ... The official basis period as defined by HMRC is 6 April to 5 April each year (the same as the tax year). So if you were self-employed for the entire 2018/2019 tax year your basis period is 6 April 2018 to 5 April 2019. HMRC Basis Period or Accounting Period . Accounting periods are often confused with the basis period. But there is a difference.