Here in the UK, the tax year runs from April to April. Your employer reports exactly your taxable earnings to the government every month. You can log in online and see exactly what you're taxed on. You do not need to fill out a self-assessment at all - it's all done automatically, except for under certain circumstances which don't affect most of the population, but roughly:
* You have earnings over £1000 a year from outside of your normal employment (e.g. from interest on savings, selling things on eBay, whatever). * You are earning above £50,000 per year but want to continue claiming child benefit (which phases out between £50,000 and £60,000, so some needs to be repaid). * You earn over £100,000.