due date vs timespan

due date

noun
  • The date by which a task is supposed to be completed. 

  • The date on which a pregnant woman is expected to give birth. 

  • The date by which a book (or other item) borrowed from a library must be returned. 

  • The date of payment in financial arrangements. 

timespan

noun
  • An interval of time. 

How often have the words due date and timespan occurred in a corpus of books? (source: Google Ngram Viewer )