A Self Assessments is a compulsory tax return issued by HM Revenue & Customs after the end of the tax year i.e. 5th April. This return is a declaration of your earnings throughout the financial year and must be submitted by the 31st January the following year.The constant stream of changes to tax legislation means not only that the returns are becoming more difficult to understand but also that taxpayers risk even more difficult incurring more penalties through failing to complete their returns on time or accurately.