TSQL Determine every other Friday from a "seed" date - sql-server

Question

Greetings StackOverflow Wizards.
SQL datetime calculations always give me trouble. I am trying to determine if an employee's hiredate fell between the last payday of that month and the first of the next month. (I.e. did they get a paycheck in their hire month.
Knowns:
I know our paydays are every other Friday.
I know 01/02/1970 was a Payday, and that date precedes the longest
active employee we have.
I know the hire date of each active employee (pulled from table).
I know (can calculate) the first of the month following the hire
date.
What I cannot seem to wrap my head around is how to use that seed date (01/02/1970) with datediff, dateadd, datepart, etc. to determine if there is a pay date between the hire date in question and the first of the following month.
In pseudo-code, here is what I'm trying to do:
declare #seedDate datetime = '01/02/1970' -- Friday - Payday seed date from which to calculate
declare #hireDate datetime = '09/26/2008' -- this date will actually be pulled from ServiceTotal table
declare #firstOfMonth datetime = DATEADD(MONTH, DATEDIFF(MONTH, 0, #hireDate) + 1, 0) -- the first of the month following #hireDate
declare #priorPayDate datetime -- calculate the friday payday immediately prior to #firstOfMonth
if #priorPayDate BETWEEN #hireDate AND #firstOfMonth
begin
-- do this
end
else
begin
-- do that
end
Using the hard-coded #hireDate above, and the #seedDate to determine every-other-Friday paydays, I know that there was a payday on 9/19/2008 and not another one until 10/03/2008, so the boolean above would be FALSE, and I will "do that" rather than "do this." How do I determine the value of #priorPayDate?

Answer 1

In all my databases where there is a lot going on with dates I create a table with colums for date,day, weekday,month,weeknr,dayof month, etc etc. I then use a procedural programming language or a bunch of handwritten sql to populate this table with every day for a large range of years say 1970 to 2200.
I pack this table 100% and index it heavily. You can then simply join any date to this table and do complex date stuff with simple where clause. So basically you pre calculate a helper table. maybe in you case you add a column to the date helper table with friday since seed column.
hope that makes sense.

Answer 2

Taking a DATEDIFF for days between your #seedDate and #firstOfMonth will give you a total number of days, which you can modulus by number of days between pay periods (14) to get number of days from the last pay period to the #firstOfMonth. You'll run into problems when the 1st is a payday (e.g. next month), which makes a CASE statement necessary:
DECLARE #priorPayDate DATETIME
SET #priorPayDate = CASE
WHEN DATEDIFF(dd, #seedDate, #firstOfMonth) % 14 = 0
THEN DATEADD(dd, -14, #firstOfMonth)
ELSE DATEADD(dd, -(DATEDIFF(dd, #seedDate, #firstOfMonth) % 14), #firstOfMonth)
END