Sentence:  If you were required to file a declaration of estimated tax for the calendar year 1961 , it is not <WSD>necessary</WSD> to pay the fourth installment otherwise due on January 15 , 1962 , if you file your income tax return Form 1040 , and pay your tax in full for the calendar year 1961 by January 31 , 1962 . 
Meaning of the word marked with <WSD> tags:  absolutely essential