Convert Informix Stored Procedure

I would like to convert a couple informix stored procedures to SQL
Server stored procedures. I have no idea how to accomplish this. Here
is an example of one of the procedures I need to convert.

drop function mnaf_calc_calendar_quarter;

CREATE FUNCTION mnaf_calc_calendar_quarter(pEndDate Date)
--************************************************** ***************************
-- Name: mnaf_calc_calendar_quarter
-- Description/Notes:
-- Calculates the most recent calendar quarter based on the end date.
--
-- Parms:
-- End Date.
-- Returns:
-- The calculated period start date and end date.
--
--************************************************** ***************************
-- Revisions:
-- PgmrDate # Description
-- HPI05/03/2005
--************************************************** ***************************

RETURNING date, date;

DEFINE dtStartDate date;
DEFINE dtEndDate date;

LET dtStartDate = mdy(12,31,1899);
LET dtEndDate = pEndDate;

-- If the end date parameter is equal to a calendar quarter,
-- calculate the start date by subtracting three months.
IF month(pEndDate) = 3 or month(pEndDate) = 6 or
month(pEndDate) = 9 or month(pEndDate) = 12 then

LET dtEndDate = pEndDate;

ELSE

-- Otherwise find the closest previous calendar quarter end date
-- then calculate the start date.
IF month(pEndDate) = 1 or month(pEndDate) = 4 or
month(pEndDate) = 7 or month(pEndDate) = 10 then

-- Subtract 1 month off end date parameter to get the calendar
qtr end date
LET dtEndDate = mnaf_eomonth(mnaf_bomonth(dtEndDate) - 1 units
month);

ELSE

-- Month must be equal to 2, 5, 8, 11
-- Subtract 2 months off end date parameter to get the calendar
qtr end date
LET dtEndDate = mnaf_eomonth(mnaf_bomonth(dtEndDate) - 2 units
month);

END IF;

END IF;

-- Calcuate the start date by subtracting off two months
LET dtStartDate = (mnaf_bomonth(dtEndDate) - 2 units month);

RETURN dtStartDate, dtEndDate;

END FUNCTION;

grant execute on mnaf_calc_calendar_quarter to public;