SQL Server 2008 :: How To Get Column From Another Table And Insert Into Other Table

I have two table 'Cal_date' and 'RPT_Invoice_Shipped'.Table cal_data has column month_no, start_date and end_date. And table RPT_Invoice_Shipped has columns Day_No, Date, Div_code, Total_Invoiced, Shipped_Value, Line_Shipped, Unit_Shipped, Transaction_Date.

I am using below insert statment to insert data in RPT_Invoice_Shipped table.

insert into [Global_Report_Staging].[dbo].[RPT_Invoice_Shipped]
(Day_No, Date, Div_code, Total_Invoiced, Transaction_Date)
select , CONVERT(DATE,Getdate()) as Date, LTRIM(RTRIM(div_Code)),
sum(tot_Net_Amt) as Total_Invoiced, (dateadd(day, -1, convert(date, getdate())))
from [Global_Report_Staging].[dbo].[STG_Shipped_Invoiced]
WHERE CONVERT(DATE,Created_date )=CONVERT(DATE,Getdate())
group by div_code

while inserting in column Day_No in RPT_Invoice_Shipped table, I have to use formula (Transaction_Date-start_date+1) where Transaction_Date from STG_Shipped_Invoiced and start_date from Cal_date table. I was using datepart (mm, Transaction_Date) so it gives month_no, and this month_no we can join with month_no of Cal_date table and fetch start_date from Cal_date table, so that we can use start_date for formula (Transaction_Date-start_date+1).

But I am getting difficulty to arrange this in above query. how to achieve this?