So this year-end tax adjustment thing is basically tax reconciliation you do as an employee with your employer who withholds your income tax from your paycheck. If I understand this correctly, the tax adjustment is just for your job. So if you have any other income – or want deductions not covered by the one filed by your employer – you'd still need to file a separate tax return.
As for the deduction certificate – again, if I understand this correctly – it's just a kind of common supporting documentation for a tax deduction. For instance, you'd get one from your insurance. The idea is that they're certificates specifically for claiming a tax deduction, as opposed to being simply proof of payment for something that happens to be eligible for a tax deduction.
And March 15th is the deadline for filing the previous year's tax returns.
