查找给定周数周期的最接近的未来日期

Question

Let's say I have a:

start_date = '2020-01-06'

and date(now())

And I want to find closest future date, starting from now that is a multiplication of 4 (n) (or another number) weeks cycle from start_date

Expected output as for today would be '2020-03-02'

I can't came up with any smart idea to check for it.

I'll highly appreciate any suggestions.

Answer 1

You could do date arithmetics:

• compute the difference between now and the "anchor" date
• divide that by 7 (days per week) and 4 (weeks)
• take the next integer value from the result
• multiply by 4 and 7, then add that number of days to the current date

In SQL:

date'2020-01-06' + ceil((current_date - date'2020-01-06')::numeric / 4 / 7)::int * 4 * 7

Dremo on DB Fiddlde :

with t as (select date'2020-01-06' anchor, 4 n)
select 
    anchor,
    n,
    current_date,
    anchor + ceil((current_date - anchor)::numeric / n / 7)::int * n * 7 next_date
from t

anchor     |  n | current_date | next_date 
:--------- | -: | :----------- | :---------
2020-01-06 |  4 | 2020-02-10   | 2020-03-02