Below script will calculate total number of days based on the input from user from 1st Jan. Though we have a built in function DATEDIFF from which we can calculate this easily. But in the below script, i have tried calculating the days without this built in function.

I have used Scalar UDF for calculating remaining days in the month. Script for the same can be found in my first article.

/* Calculate total number of days from 1st Jan */

declare @fresult integer
declare @tdate date
declare @mdate integer
declare @counter integer
declare @intermidate_result integer
set @tdate = '2012-12-31'
set @counter = 1
set @mdate = MONTH(@tdate)
set @intermidate_result = 0
set @fresult = 0
while @counter < @mdate
begin
/* below function will give last month remaining days */
set @fresult = dbo.udf_calculate_remaining_days(DATEADD(mm,-@counter,@tdate))
/* will give total numbers of days minus current month */
set @intermidate_result = DATEPART(dd,dateadd(mm,-@counter,@tdate))+ @fresult+@intermidate_result
set @counter = @counter + 1
end
declare @current_date integer
set @current_date = DAY(@tdate)
set @intermidate_result = @intermidate_result + @current_date
select 'Total Number Of Days from :'+'2012-01-01'+'  '+'To'+'  '+CAST(@tdate as varchar(10))+'  '+'is:'+'  '+CAST(@intermidate_result as varchar(4))

Sample Output:
Total Number Of Days from :2012-01-01  To  2012-12-31  is:  366