Why do different date math operations produce the same value?

Question

I can add six days to a date and get what I expect, but if I add seven days to that same date, I get the same date.

What I mean is this; when the user has enter "12/27/2015" as the value for the @BegDate parameter, this declaration:

DECLARE 
@Week1End datetime = DATEADD(Day, 6, @BegDate),
@Week2Begin datetime = DATEADD(Day, 7, @BegDate);

...results in both @Week1End and @Week2Begin being the same date (2016-01-02), as you can see from the result set returned:

This is the TSQL:

CREATE PROCEDURE [dbo].[variancePriceByProductWithPriceChangePercentage]
    @Unit varchar(25),
    @BegDate datetime,
    @EndDate datetime
AS

DECLARE 
@Week1End datetime = DATEADD(Day, 6, @BegDate),
@Week2Begin datetime = DATEADD(Day, 7, @BegDate);

Select Ind.Description,
    @BegDate  BegDate,
    @Week1End Week1End,
    @Week1End Week2Begin,
    @EndDate EndDate,
    SUM(CASE WHEN Ind.InvoiceDate BETWEEN @BegDate AND @Week1End THEN Ind.QtyShipped ELSE 0 END) 
Week1Usage,
    SUM(CASE WHEN Ind.InvoiceDate BETWEEN @Week2Begin AND @EndDate THEN Ind.QtyShipped ELSE 0 END) 
Week2Usage,
    SUM(CASE WHEN Ind.InvoiceDate BETWEEN @BegDate AND @Week1End THEN Ind.Price ELSE 0 END) 
Week1Price,
    SUM(CASE WHEN Ind.InvoiceDate BETWEEN @Week2Begin AND @EndDate THEN Ind.Price ELSE 0 END) -
   SUM(CASE WHEN Ind.InvoiceDate BETWEEN @BegDate AND @Week1End THEN Ind.QtyShipped ELSE 0 END) 
UsageVariance,
    SUM(CASE WHEN Ind.InvoiceDate BETWEEN @Week2Begin AND @EndDate THEN Ind.QtyShipped ELSE 0 END) 
-
    SUM(CASE WHEN Ind.InvoiceDate BETWEEN @BegDate AND @Week1End THEN Ind.Price ELSE 0 END) 
PriceVariance,
    (SUM(CASE WHEN Ind.InvoiceDate BETWEEN @Week2Begin AND @EndDate THEN Ind.QtyShipped ELSE 0 
END)  -
    SUM(CASE WHEN Ind.InvoiceDate BETWEEN @BegDate AND @Week1End THEN Ind.Price ELSE 0 END) )
    / 
NULLIF(SUM(CASE WHEN Ind.InvoiceDate BETWEEN @BegDate AND @Week1End THEN Ind.Price ELSE 0 END),0) 
PercentageOfPriceVariance
    From    InvoiceDetail Ind
    Where   Ind.Unit = @Unit
    AND Ind.InvoiceDate BETWEEN @BegDate AND @EndDate
    Group By Ind.Description

So why does @BegDate + 6 equate to the same thing as @BegDate + 7

Answer 1

@Week1End Week1End,
@Week1End Week2Begin,

Look at this piece of code again...