calculate no of days at report and calculations required more than 180 will be 0 for next year

I am receiving start date and End date from source system.  I
need to calculate number of days. ?

Formula for No. Days as follows

1. No. Days = End Date – Start Date (If the end date
   value is available),  else last day of the current month – Start Date (If end
   date values is blank).

                      If the number of days > 180 ( I need
to show 180 days only for that year), if the year is changing then number of
days should be zero.