Is there a formula or function for Numbers that returns the number of days or weeks for a given year
Try using the DATEDIF(start-date,end-date,calc-method) divided by 7 to determine number of weeks.

For example:



DATEDIF returns the number of days between to dates.  Dividing that by 7 should result in the number of weeks.  This can be rounded as desired and calculated against.

From the Formula reference:


  The DATEDIF function returns the number of days, months, or years between two dates.
  DATEDIF(start-date, end-date, calc-method)
  start-date: The starting date. start-date is a date/time value (the time portion is ignored) or date string value.
  end-date: The ending date. end-date is a date/time value (the time portion is ignored) or date string value.
  calc-method: A modal value that specifies how to express the time difference and how dates in different years or months are handled.
  “D”: Return the number of days between the start and end dates.
  “M”: Return the number of months between the start and end dates.
  “Y”: Return the number of years between the start and end dates.
  “MD”: Return the days between the start and end dates, ignoring months and years. The month in end-date is considered to be the month in start-date. If the starting day is after the ending day, the count starts from the ending day as if it were in the preceding month. The year of the end-date is used to check for a leap year.
  “YM”: Return the number of whole months between the start and end dates, ignoring the year. If the starting month/day is before the ending month/day, the dates are treated as though they are in the same year. If the starting month/day is after the ending month/day, the dates are treated as though they are in consecutive years.
  “YD”: Return the number of days between the start and end dates, ignoring the year. If the starting month/day is before the ending month/day, the dates are treated as though they are in the same year. If the starting month/day is after the ending month/day, the dates are treated as though they are in consecutive years.