WARNING! slight syntax difference. See how to use. SQL server has dateadd and so should postgres. Run the following DDL and have date add in your custom user defined functions.

postgres_dateadd.sql

CREATE OR REPLACE FUNCTION  dateadd(_timelabel VARCHAR(25),_timevalue NUMERIC,_timegiven TIMESTAMP)
RETURNS TIMESTAMP AS
$BODY$
DECLARE returnVal TIMESTAMP;
/*
-- =============================================
-- Author     : Daniel Van Den Bosch
-- Create date: 8/20/2018
-- Description: Use dateadd (a ms sql server function) in postgres
-- How To use : Your are going to have to add quotes to the _timelabel AND it is a good idea to cast :: TO TIMESTAMP _timegiven::TIMESTAMP
                Take the following example: MS TSQL  : SELECT DATEADD(year, 1, current_timestamp) AS DateAdd;
--                                          POSTGRES : SELECT DATEADD('year', 1, current_timestamp::TIMESTAMP) AS DateAdd;
-- =============================================
___________________________________________
|datepart	   | Abbreviations  | supported  |
-------------------------------------------|
|year	       | yy, yyyy       |     x      |
-------------------------------------------|
|quarter	   | qq, q          |     x      |
-------------------------------------------|
|month	     | mm, m          |     x      |
-------------------------------------------|
|dayofyear	 | dy, y          |    no      |
-------------------------------------------|
|day	       | dd, d          |     x      |
-------------------------------------------|
|week	       | wk, ww         |     x      |
-------------------------------------------|
|weekday	   | dw, w          |     no     |
-------------------------------------------|
|hour	       | hh             |     x      |
-------------------------------------------|
|minute	     | mi, n          |     x      |
-------------------------------------------|
|second	     | ss, s          |     x      |
-------------------------------------------|
|millisecond | ms             |     x      |
-------------------------------------------|
|microsecond | mcs            |     no     |
-------------------------------------------|
|nanosecond	 | ns             |     no     |
-------------------------------------------|
 */
DECLARE

BEGIN
IF _timelabel='yy' OR _timelabel='yyyy' OR _timelabel = 'year' OR _timelabel='years' THEN
returnVal=_timegiven + (_timevalue::TEXT || ' years')::INTERVAL;
ELSIF _timelabel='qq' OR _timelabel='q' OR _timelabel = 'quarter' OR _timelabel='quarters' THEN
returnVal=_timegiven + ((_timevalue*3)::TEXT || ' months')::INTERVAL;
ELSIF _timelabel='mm' OR _timelabel='m' OR _timelabel = 'month' OR _timelabel='months' THEN
returnVal=_timegiven + ((_timevalue)::TEXT || ' months')::INTERVAL;
ELSIF _timelabel='dd' OR _timelabel='d' OR _timelabel = 'days' OR _timelabel='day' THEN
returnVal=_timegiven + ((_timevalue)::TEXT || ' days')::INTERVAL;
ELSIF _timelabel='wk' OR _timelabel='ww' OR _timelabel = 'week' OR _timelabel='weeks' THEN
returnVal=_timegiven + ((_timevalue)::TEXT || ' week')::INTERVAL;
ELSIF _timelabel='hh' OR _timelabel='h' OR _timelabel = 'hour' OR _timelabel='hours' THEN
returnVal=_timegiven + ((_timevalue)::TEXT || ' hours')::INTERVAL;
ELSIF _timelabel='mi' OR _timelabel='n' OR _timelabel = 'minute' OR _timelabel='minutes' THEN
returnVal=_timegiven + ((_timevalue)::TEXT || ' minutes')::INTERVAL;
ELSIF _timelabel='ss' OR _timelabel='s' OR _timelabel = 'second' OR _timelabel='seconds' THEN
returnVal=_timegiven + ((_timevalue)::TEXT || ' seconds')::INTERVAL;
ELSIF _timelabel='ms' OR _timelabel='ms' OR _timelabel = 'millisecond' OR _timelabel='milliseconds' THEN
returnVal=_timegiven + ((_timevalue)::TEXT || ' milliseconds')::INTERVAL;
ELSE
returnVal='1/1/1900'::TIMESTAMP;
END IF;
--raise exception 'dateadd interval parameter not supported';
return returnVal;
END;
$BODY$
LANGUAGE 'plpgsql';