When posting a work, you have the option to set a different publication date — in other words, to backdate the work. You can also set publication dates for individual chapters. Please note that in both cases, this affects whether and where your work appears on your dashboard and the works page, etc. These pages which show the date when your work was last updated — the publication date of your work or of one of your chapters, whichever is the most recent date.
Any subsequent chapters you add will have that date pre-set on the form, though you will still have the option of overriding that date. This means that if you don't know or care exactly when each chapter was originally published, you can still backdate your work conveniently.