Calculate last working date in a month

Hello guys.
I have some trouble using CalcDateBOC and CheckDateStatus functions in codeunit 7600. I am not sure how to use them.
How can I calculate what is the last working date in a month?
For example, if customer validates payment date to be 12.1.2017. then in a field "Last date" it should validate 31.1.2017..
Thanks