Sentence:  However , if their United States income is <WSD>not</WSD> subject to the withholding of tax on wages , their returns are due June 15 , 1962 , if they use a calendar year , or the 15 th day of the 6 th month after the close of their fiscal year . 
Meaning of the word marked with <WSD> tags:  negation of a word or group of words