how to divide actual months to 5 days periods using plpgsql function - postgresql

Question

I need to get first day of each five days of month using posgrees. For example if no is 2.03 then I would get 1.03. If today would be 19.03 then I would get 15.03. I tried to use following if statements but for today;s day I got null and I'm not sure why. Also there surely is some less complicated way to do this operation. Any ideas?
create or replace function getFirstDayOfFive()
returns timestamp with time zone as $$
declare
firstDay timestamp;
begin
if (now()::date>date_trunc('month', now()::date) and now()::date < date_trunc('month', now()::date) + interval '5 day') then
return firstDay = date_trunc('month', now()::date);
elsif (now()::date>date_trunc('month', now()::date) + interval '5 day' and now()::date < date_trunc('month', now()::date) + interval '10 day') then
return firstDay = (date_trunc('month', now()::date) + interval '5 day')::date;
elsif (now()::date>date_trunc('month', now()::date) + interval '10 day' and now()::date < date_trunc('month', now()::date) + interval '15 day') then
return firstDay = (date_trunc('month', now()::date) + interval '10 day')::date;
elsif (now()::date>date_trunc('month', now()::date) + interval '15 day' and now()::date < date_trunc('month', now()::date) + interval '20 day') then
return firstDay = (date_trunc('month', now()::date) + interval '15 day')::date;
elsif (now()::date>date_trunc('month', now()::date) + interval '20 day' and now()::date < date_trunc('month', now()::date) + interval '25 day') then
return firstDay = (date_trunc('month', now()::date) + interval '20 day')::date;
end if;
end;
$$
language plpgsql;

Answer 1

demo:db<>fiddle
SELECT
make_date( -- 4
date_part('year', my_date)::int, -- 3
date_part('month', my_date)::int,
greatest( -- 2
floor(date_part('day', my_date) / 5) * 5, -- 1
1
)::int
)
Get day of the current date using date_part(). After it round it to full multiple 5 with floor(day / 5) * 5
The provided algorithm always gives the previous multiple of 5 (for 19 it gives 15, for 6 it gives 5, ...). However, for days 1 to 4 it gives 0. So this is an exception which needs to be handled. This is done here
Get year and month part of the current date
Create the expected date.
This query can be embedded into a function, of course:
create or replace function getFirstDayOfFive()
returns timestamp with time zone as $$
declare
firstDay timestamp;
begin
SELECT
make_date(
date_part('year', now())::int,
date_part('month', now())::int,
greatest(
floor(date_part('day', now()) / 5) * 5,
1
)::int
)
INTO firstDay;
RETURN firstDay;
end;
$$
language plpgsql;
Edit: From the comments: Same for Last Day:
You have to change:
floor() to ceil()
the greatest(..., 1) into least(..., last day of current month)
To get the last date of the current month, you have to find the first day using date_trunc('month', ...), add one month to get the first day of the next month and subtract one day from it:
demo:db<>fiddle
SELECT
least(
ceil(date_part('day', now()) / 5) * 5,
date_part('day', date_trunc('month', now()) + interval '1 month - 1 day')
)