A date-handling module is being designed. Sort each item by whether it should be public or private.

Public interface: A function that formats a date as a string, A function that reports whether a year is a leap year, A function that adds a number of days to a date

Private internals: The lookup array of month lengths used internally, A helper that converts a date into a day count since 1970, The scratch buffer the formatter reuses between calls

The public interface should be small and stated in terms of dates, not of the machinery that manipulates them: format, leap-year test and date arithmetic. Month-length tables, internal day counts and scratch buffers exist only to make those work, and publishing them would let callers couple to details you wanted to be free to change.