Declare Parameters in Teradata query OR If-then-else - if-statement

Question

I am new to Teradata. Here is the situation:
If Current_date is < 15 of month then ?Startdate parameter = 1st of prev month and ?EndDate param = last date of current month
SELECT ... FROM ViewA
WHERE date BETWEEN ?Startdate AND ?EndDate
ELSE if Current_date >= 15 then Startdate parameter = 1st of Current month and EndDate param = last date of current month
SELECT ... FROM ViewA
WHERE date BETWEEN ?Startdate AND ?EndDate
I was able to calculate dates in each case, but can't figure out:
#1) how to put them in parameters. If I can put them in params then all I have to do is
SELECT ... FROM ViewA
WHERE date BETWEEN ?Startdate AND ?EndDate
OR
#2) Write a IF-THEN-ELSE
If Current_date is < 15 of month
SELECT ... FROM ViewA
WHERE Date BETWEEN
ADD_MONTHS((DATE - EXTRACT(DAY FROM DATE)+1), -1) AND ADD_MONTHS(LAST_DAY(DATE),0)
ELSE
SELECT ... FROM ViewA
WHERE date BETWEEN
ADD_MONTHS((DATE - EXTRACT(DAY FROM DATE)+1), 0) AND ADD_MONTHS(LAST_DAY(DATE),0) )
END IF
Please guide how to achieve #1 or #2. I CANNOT create function or stored procedure in teradata

Answer 1

#2 using calendar as an example.
SELECT calendar_date from Sys_Calendar.CALENDAR
WHERE calendar_date
BETWEEN
case when extract (day from current_date) < 15
then ADD_MONTHS((current_date - EXTRACT(DAY FROM current_date)+1), -1)
else current_date - EXTRACT(DAY FROM current_date)+1
end
AND ADD_MONTHS(LAST_DAY(current_date),0)
order by calendar_date
;
I prefer the current_date, can't be confused with the datatype.

Answer 2

WHERE date
BETWEEN Trunc(current_date - 14 , 'mon') -- first day of the month 14 days before today
AND last_day(current_date) -- last day of current month