Determine next b-weekly date

I need to determine what the next day of the month is that falls exactly on a two week interval from a given date  

 Example...
 A person has a payment to make
 First Payment was on July 20th 2010
 Interval is Bi-Weekly (Every 2 weeks)
When is the next date that two week interval would occur on?
Larry Bristersr. DeveloperAsked:
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pcelbaConnect With a Mentor Commented:
This you may calculate very simply as a remainder when you divide the number of days by 14:

DECLARE @payCheck Date
SET @payCheck = '2010-07-20'  

SELECT DATEDIFF(day, @payCheck, GETDATE()) AS NoOfDays,
              CASE DATEDIFF(day, @payCheck, GETDATE()) % 14 WHEN 0 THEN 'YES' ELSE 'no' END AS  'PayDateToday'
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Jim HornMicrosoft SQL Server Developer, Architect, and AuthorCommented:
That would be DATEADD

SELECT DATEADD(d, 14, FirstPaymentDate) as NextPaymentDate
FROM YourTable

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Jim HornMicrosoft SQL Server Developer, Architect, and AuthorCommented:
Is your ultimate requirement to create rows for an entire time series with rows 14 days apart, or just get the next one?
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Tapan PattanaikSenior EngineerCommented:
Hi lrbrister,

SELECT GETDATE() 'Today', DATEADD(week,2,GETDATE()) 'Today + 2 Weeks';

SELECT GETDATE() 'Today', DATEADD(week,2,GETDATE())  +1 'Today + 2 Weeks + oneDay';

Regards,
Tapan Pattanaik
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pcelbaCommented:
I would extend previous answers by the first payment exact date specification:

DECLARE @payCheck Date
SET @payCheck = '2016-07-20'  

SELECT @payCheck AS PayCheck, DATEADD(week, 2, @payCheck) AS NextDate
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Larry Bristersr. DeveloperAuthor Commented:
Hey guys...
The "PaymentDate" is actually the FIRST PaymentDate

So...
On Bi-Weekly... that started on a Tuesday on July 2 2010
Does TODAY  fall on a every two weeks date

Because it's every 14 days... it will of course very seldom be on the 2nd, the 16th and the 30th every month.
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Jim HornMicrosoft SQL Server Developer, Architect, and AuthorCommented:
In that case you're going to need some kind of a loop that begins at the start date and ends at whatever the term date is.   So for how long is the term?  One year, 30 years, ... ?

Check out SQL Server Calendar Table and do a control-Find on 'WHILE @i <= @total_days'.  You'll need to do something like this, either inserting rows into a temporary table and then selecting that table at the end of your code, or just inserting.
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pcelbaCommented:
When On Bi-Weekly... that started on a Tuesday on July 20 2010
Does TODAY  fall on a every two weeks date?

No. Because today is Wednesday.
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Larry Bristersr. DeveloperAuthor Commented:
pcelba Yeah... but if today was the same day of the week as July 20 2010 BUT it was not an even multiple of 14 days difference  it would still be a no
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hnasrConnect With a Mentor Commented:
Check this: This code checks today's date and calculates the number of weeks passed.
Next date is determined by checking the number of weeks, odd or even.
declare @dt datetime	--first date
declare @today as datetime -- todays' date
declare @w int -- weeks from start date to today
declare @n datetime -- next date in bi weekly interval
select @dt='2016-07-20'; -- example
select @today=sysdatetime();

select sysdatetime(), @dt;

SELECT dateadd(WEEK,2,@dt);
select @w=DATEDIFF(WEEK,@dt,@today)
if @w=(@w/2)*2
	SELECT @n=dateadd(WEEK,@w,@dt)
ELSE
	SELECT @n=dateadd(WEEK,(@w/2)*2,@dt)

	SELECT @dt, @n, @w

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Larry Bristersr. DeveloperAuthor Commented:
Hey guys...
I kind of blended everything you guys had and came up with this function.
It seems to work fine... you folks see anything glaringly wrong?


USE [EverywareV3]
GO

/****** Object:  UserDefinedFunction [dbo].[IsPaymentDate]    Script Date: 11/2/2016 7:21:30 PM ******/
SET ANSI_NULLS ON
GO

SET QUOTED_IDENTIFIER ON
GO

-- ======================================================================================
-- Author:		lbrister-- Create date: 10.26.2016
-- Description:	Determine if today is payment date
-- SELECT dbo.IsPaymentDate('Weekly','09/28/2016')
-- SELECT dbo.IsPaymentDate('Monthly','09/28/2016')
-- SELECT dbo.IsPaymentDate('Bi-Weekly','10/12/2016')
-- SELECT dbo.IsPaymentDate('Bi-Monthly','09/28/2016') --Returns yes on 1 and 15th
-- ======================================================================================
ALTER FUNCTION [dbo].[IsPaymentDate]
	   (
		@Interval VARCHAR(12) ,
		@PaymentDate DATETIME
	   )
RETURNS VARCHAR(10)
AS
	BEGIN
	-- Declare the return variable here
		DECLARE	@ResultVar VARCHAR(10) = '';

		DECLARE	@Today DATETIME = CONVERT(VARCHAR(10), GETDATE(), 101);
		DECLARE	@PaymentDayOfWeek INT = DATEPART(dw, @PaymentDate);
		DECLARE	@CurrentDayOfWeek INT = DATEPART(dw, GETDATE());
		DECLARE	@BiWeeklyDate BIT = DATEDIFF(d, @PaymentDate, CAST(GETDATE() AS DATE)) % 14;

		SET @ResultVar = (
						  SELECT	CASE WHEN @Interval IN ('Weekly')
											  AND @CurrentDayOfWeek = @PaymentDayOfWeek THEN 'YES'
										 WHEN @Interval IN ('Monthly')
											  AND DAY(@PaymentDate) = DAY(GETDATE()) THEN 'YES'
										 WHEN @Interval = 'Bi-Weekly'
											  AND @BiWeeklyDate = 0 THEN 'YES'
										 WHEN @Interval = 'Bi-Monthly'
											  AND DAY(GETDATE()) IN (1, 15) THEN 'YES'
										 ELSE 'NO'
									END
						 );

	-- Return the result of the function
		RETURN @ResultVar;

	END;

GO

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Larry Bristersr. DeveloperAuthor Commented:
Sorry for the late get back.
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