Using T-SQL DATEADD and DATEDIFF function - sql-server-2008

Question

I am trying to understand the code that someone wrong to implement the following requirements in calculating a date:
The derived JE_ACCTNG_DT is set equal to the last day of the prior month of the current year when the feed is created and sent.
For example, if the current year is 2013, the JE_ACCTNG_DT will be the following based on the quarter the feed is sent:
JE_ACCTNG_DT = 3/31/2013 for 1st quarter feed sent on 4/1/2013
JE_ACCTNG_DT = 6/30/2013 for 2nd quarter feed sent on 7/1/2013
JE_ACCTNG_DT = 9/30/2013 for the 3rd quarter feed sent on 10/1/2013
JE_ACCTNG_DT = 12/31/2013 for the 4th quarter feed sent on 1/1/2014
I see that someone implemented the code like this and it seems to give the right answer for the examples given in the requirements, if the datetime result is converted to a date data type (the time portion is dropped)
DECLARE #FEED_DT datetime
SET #FEED_DT = '4/1/2013'
select DATEADD(s,-1,DATEADD(mm, DATEDIFF(m,0,#FEED_DT),0))
SET #FEED_DT = '7/1/2013'
select DATEADD(s,-1,DATEADD(mm, DATEDIFF(m,0,#FEED_DT),0))
--2013-06-30 23:59:59.000
SET #FEED_DT = '10/1/2013'
select DATEADD(s,-1,DATEADD(mm, DATEDIFF(m,0,#FEED_DT),0))
--2013-09-30 23:59:59.000
SET #FEED_DT = '1/1/2014'
select DATEADD(s,-1,DATEADD(mm, DATEDIFF(m,0,#FEED_DT),0))
--2013-09-30 23:59:59.000
What is confusing me is that the DATEDIFF function accepts the following arguments: A datepart and two dates
DATEDIFF (datepart ,startdate ,enddate )
If you notice above, the code set the date part to "m" but passed zero for the 2nd parameter when a date is expected.
DECLARE #FEED_DT datetime = '4/1/2013'
DECLARE #DateAddResult as int
SET #DateAddResult = DATEDIFF(m,0,#FEED_DT)
select #DateAddResult
--1359
select DATEADD(s,-1,DATEADD(mm, #DateAddResult,0))
--2013-03-31 23:59:59.000
What is the code really doing? I would've expected an error. Did it coerce the 0 value to a date?
I'm thinking that what the requirements should really be is to calculate the last day of the previous quarter. This is how I would do it.
SELECT dateadd(day, -1, DATEADD(quarter,DATEDIFF(quarter,'1900/01/01', GETDATE()), '1900/01/01')) as FirstDayOfQuarter

Answer 1

Given this:
select DATEADD(s,-1,DATEADD(mm, DATEDIFF(m,0,#FEED_DT),0))
Let's take it one piece at a time...
DATEDIFF(m,0,#FEED_DT)
...calculates the number of whole integer months that has alapsed from date value zero (which is evaluated as 1-1-1900) to the date value started in the datetime field #FEED_DT. Let's say that this value is #ElapsedMonthsSinceZero and replace this variable into the original nested function, giving:
select DATEADD(s,-1,DATEADD(mm, #ElapsedMonthsSinceZero,0))
Now let's evaluate this part:
DATEADD(mm, #ElapsedMonthsSinceZero,0)
This adds the same number of months to the Date "zero", i.e. 1-1-1900, which brings you to the first day of the month of the date #FEED_DT. Essentially, what we did so far was calculate the whole number of months that have elapsed from date 0, truncating off any day beyond day 1 into the month. This gives us #FirstDayOfFEED_DT. Replacing that into the function gives us...
select DATEADD(s,-1,#FirstDayOfFEED_DT)
Now that we have the first day of the month of the date #FEED_DT, subtracting 1 gives us the last day of the previous month.
I guess I was just thrown off by the discrepancy where a value of 0 was passed instead of a date.

Answer 2

suppose you add some days in a new column and you hava a stored procedure of a parameter #date and you want to add current date to that parameter and get result in days.
DateAdd(days,#date,Getdate())