sql - Sum the differences between dates -

i have 2 tables, master , detail, both contain dates related events. first contain master record's begin , end date. second contains various gaps, have beginning , end, related master record , falling between begin , end date. can calculate total number of days between master record's start , end but, yet i'm failing see how can aggregate sum of "off days" in details table , group them month. mean:

master table

start date (mm/dd/yyyy): 01/01/2015 end date (mm/dd/yyyy): 01/25/2015 

total number of days: 25

details table

start date (mm/dd/yyyy) | end date (mm/dd/yyyy) :  01/02/2015              | 01/05/2015  01/09/2015              | 01/15/2015 01/18/2015              | 01/19/2015 

total number of "off days": 13

the db environment oracle 11g. can me?

try :

select sum(end_date-start_date+1) details_table;  

the sum function sum total of dates, should give 13 "off days" wanted.

if want add start_date/end_date conditions, can this.

select sum(end_date-start_date+1) details_table  start_date>=to_date('01/01/2015','mm/dd/yyyy')  , end_date<=to_date('01/25/2015','mm/dd/yyyy');