mean solar day vs working day

mean solar day

noun
  • the average length of a solar day, used for practical timekeeping 

working day

noun
  • The part of a day in which work is done; the number of hours one must work per day for a specified wage. 

  • Any of those days of a week on which work is done, officially Monday to Friday in many countries (even though many people work on weekends). 

How often have the words mean solar day and working day occurred in a corpus of books? (source: Google Ngram Viewer )