Another NI Voluntary Contributions query

I have not made any NI Contributions this tax year, but I do want to make the Voluntary Contributions of £741 for this tax year and the subsequent tax years until I reach State Pension age. However do I need to make the contribution for this 2017/18 tax year before 6 April? If I don't make the payment before the end of the tax year, will it appear as a shortfall of £741 for 2017/18 and allow me to pay it later?