Implentation of a date in SQL Server - sql-server

Question

Disclaimer: I am just looking for a logic not code
John discovered a strange island called Rasa. The years and weeks on the island are weird. Digging deeper into the island's calendar, he found out that it is similar to rest-of-the-world's (ROW) calendar but the island calendar's Year starts on 1st week of February's calendar. John is asking you to help him solve the problem of converting ROW's calendar into Island's calendar. Here is the question.
You are given a date (today's date). You have to determine the Island week's number. The catch here is that the Island year starts from 1st week of February and every Island's week starts from Sunday and ends on a Saturday. Write a SQL statement in SQL Server to achieve this. Use SQL Server functions and devise a logic.
Input parameter: Any date.
Output parameter: Week No in Rasa Calendar.
Here is an example:
Date: 5th May 2015 --
Week No in ROW Calendar:19
Week No in Rasa's Calendar:14
Date: Jan 1 2017:
Week No in ROW Calendar:1
Week No in Rasa's Calendar:49
My question: can this be achieved in SQL Server?
My homework: I tried a couple of ways to solve the problem.
Approach #1:
Step 1: Calculate the total no of days between today and Feb 1.
Step 2: Divide it by 7 and add 1 to the result.
Later found out that this approach will not work if Feb 1 is on any day other than Sunday.
Eg: 1st Feb is on Wednesday. 5th Feb will be on Monday
So, 1st Feb is on Rasa's week 1, and 5th Feb is on Rasa's week no 2. According to my approach 1st and 5th feb are on week 1 which is incorrect.
Approach #2:
I thought removing 5 weeks of Jan from ROW's calendar should work
select
case
when f.RasaWeek = -4 then 48
when f.RasaWeek = -3 then 49
when f.RasaWeek = -2 then 50
when f.RasaWeek = -1 then 51
when f.RasaWeek = 0 then 52
else
f.RasaWeek
end as Rasa_week,
f.year, f.month, f.date
from
(select
datepart(wk, date) - 5 as RasaWeek, *
from
<datetable>
where
Year(date) in (2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018)) as f
Info: I tested this on a <datetable> but this code will break if there is a 53rd week. Notice that I was not able to take care of the 53rd week.
Any inputs to solve this problem are welcome.

Answer 1

With dates, it's almost always easier to make a calendar table and store the data you care about rather than trying to do anything beyond basic date arithmetic. Use SQL's strengths: storing and retrieving data.
In this case, what you care about are all of the first Sundays in February. If you store these dates in a table, the solution is:
RETURN
SELECT TOP 1
DATEDIFF(day,[date],#input_date) / 7
FROM IslandCalendarStartDates
WHERE [date] <= #input
ORDER BY [date] DESC
This way you don't need to worry about leap years or 53-weeks, or any of the edge cases. Just count the days from the most recent first Sunday in February and divide by 7. If you need to change the solution to accommodate a different start date, you only change the data, not the code.