This deadline is only relevant for those individuals who complete a ‘paper’ self assessment tax return and who are employees. Where you have an underpayment you may request that HMRC collect the tax outstanding by making an adjustment to your tax code for the year 2018/19.
Please note that where your return is submitted online then the filing deadline for ‘coding out’ is 30 December 2017.
If we are already dealing with this matter on your behalf you need take no action.