Sentence:  However , if their United States income is not 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 <WSD>day</WSD> of the 6 th month after the close of their fiscal year . 
Meaning of the word marked with <WSD> tags:  time for Earth to make a complete rotation on its axis