Sentence:  These expenditures are estimated to be $ 12.1 billion , an increase of $ 187 million over 1960 , reflecting additional longevity pay of career personnel , more dependents , an increased number of men drawing proficiency pay , and social security tax increases ( effective for the full year in 1961 compared with only 6 <WSD>months</WSD> in 1960 ) . 
Meaning of the word marked with <WSD> tags:  a time unit of approximately 30 days