Created these test cases to help me further understand how to use stamp_date_time/3.
The SWI-Prolog implementation (source code) (test)
Sorry that the code is larger than allowed here, so please cut and paste it into a local editor to better see it all. 
:- begin_tests(stamp_date_time).
% GitHub SWI-Prolog library date: https://github.com/SWI-Prolog/swipl-devel/blob/10f258f755beaffecd66f488769f154be028a8ca/library/date.pl
% GitHub SWI-Prolog date test cases: https://github.com/SWI-Prolog/swipl-devel/blob/10f258f755beaffecd66f488769f154be028a8ca/src/Tests/library/test_date.pl
test(001) :-
current_prolog_flag(timezone,Timezone),
assertion( Timezone == 18000 ).
% setlocale/3 has no effect on stamp_date_time/3 that I am aware.
test(002) :-
setlocale(time,Current,Current),
assertion( Current == 'English_United States.1252' ).
% Timestamp - See comment at end of each test line on how it was generated
% | TimeZone value for stamp_date_time(_,_,+TimeZone) % Allowed values: local, 'UTC', 0 Not allowed: '-', 'Z'
% | | Expected value from stamp_date_time(_,date(_,_,_,_,_,_,Offset,_,_),_)
% | | | Expected value from stamp_date_time(_,date(Year,Month,Day,Hour,Minute,Seconds,Offset,Timezone_name,Daylight_savings_time),_)
% | | | | Year
% | | | | | Month
% | | | | | | Day
% | | | | | | | Hour
% | | | | | | | | Minute
% | | | | | | | | | Seconds
% | | | | | | | | | | Offset
% | | | | | | | | | | | Timezone name
% | | | | | | | | | | | | Daylight savings time
stamp_date_time_test_generator(1578843872.541233, local, current_timezone, date(2020, 1, 12, 10, 44, 32.54123306274414, 18000, 'Eastern Standard Time', false ) ). % Timestamp created with ?- get_time(Stamp).
stamp_date_time_test_generator(1578843872.541233, 'UTC', 0, date(2020, 1, 12, 15, 44, 32.541233062 , 0, 'UTC' , - ) ). % Timestamp created with ?- get_time(Stamp).
stamp_date_time_test_generator(1578843872.541233, 0, 0, date(2020, 1, 12, 15, 44, 32.541233062 , 0, - , - ) ). % Timestamp created with ?- get_time(Stamp).
stamp_date_time_test_generator(946702800.0 , local, current_timezone, date(2000, 1, 1, 0, 0, 0.0 , 18000, 'Eastern Standard Time', false ) ). % Timestamp created with ?- current_prolog_flag(timezone,Offset),date_time_stamp(date(2000,1,1,0,0,0,Offset ,- ,-),Stamp).
stamp_date_time_test_generator(946702800.0 , local, current_timezone, date(2000, 1, 1, 0, 0, 0.0 , 18000, 'Eastern Standard Time', false ) ). % Timestamp created with ?- date_time_stamp(date(2000,1,1,0,0,0,Offset ,local,-),Stamp).
stamp_date_time_test_generator(946702800.0 , local, current_timezone, date(2000, 1, 1, 0, 0, 0.0 , 18000, 'Eastern Standard Time', false ) ). % Timestamp created with ?- date_time_stamp(date(2000,1,1,0,0,0,Offset ,'UTC',-),Stamp).
stamp_date_time_test_generator(946702800.0 , local, current_timezone, date(2000, 1, 1, 0, 0, 0.0 , 18000, 'Eastern Standard Time', false ) ). % Timestamp created with ?- date_time_stamp(date(2000,1,1,0,0,0,Offset ,'Z' ,-),Stamp).
stamp_date_time_test_generator(946684800.0 , local, current_timezone, date(1999, 12, 31, 19, 0, 0.0 , 18000, 'Eastern Standard Time', false ) ). % Timestamp created with ?- date_time_stamp(date(2000,1,1,0,0,0,0 ,- ,-),Stamp).
stamp_date_time_test_generator(946684800.0 , local, current_timezone, date(1999, 12, 31, 19, 0, 0.0 , 18000, 'Eastern Standard Time', false ) ). % Timestamp created with ?- date_time_stamp(date(2000,1,1,0,0,0,0 ,local,-),Stamp).
stamp_date_time_test_generator(946684800.0 , local, current_timezone, date(1999, 12, 31, 19, 0, 0.0 , 18000, 'Eastern Standard Time', false ) ). % Timestamp created with ?- date_time_stamp(date(2000,1,1,0,0,0,0 ,'UTC',-),Stamp).
stamp_date_time_test_generator(946684800.0 , local, current_timezone, date(1999, 12, 31, 19, 0, 0.0 , 18000, 'Eastern Standard Time', false ) ). % Timestamp created with ?- date_time_stamp(date(2000,1,1,0,0,0,0 ,'Z' ,-),Stamp).
stamp_date_time_test_generator(946702800.0 , 'UTC', 0, date(2000, 1, 1, 5, 0, 0.0 , 0, 'UTC' , - ) ). % Timestamp created with ?- current_prolog_flag(timezone,Offset),date_time_stamp(date(2000,1,1,0,0,0,Offset ,- ,-),Stamp).
stamp_date_time_test_generator(946702800.0 , 'UTC', 0, date(2000, 1, 1, 5, 0, 0.0 , 0, 'UTC' , - ) ). % Timestamp created with ?- date_time_stamp(date(2000,1,1,0,0,0,Offset ,local,-),Stamp).
stamp_date_time_test_generator(946702800.0 , 'UTC', 0, date(2000, 1, 1, 5, 0, 0.0 , 0, 'UTC' , - ) ). % Timestamp created with ?- date_time_stamp(date(2000,1,1,0,0,0,Offset ,'UTC',-),Stamp).
stamp_date_time_test_generator(946702800.0 , 'UTC', 0, date(2000, 1, 1, 5, 0, 0.0 , 0, 'UTC' , - ) ). % Timestamp created with ?- date_time_stamp(date(2000,1,1,0,0,0,Offset ,'Z' ,-),Stamp).
stamp_date_time_test_generator(946684800.0 , 'UTC', 0, date(2000, 1, 1, 0, 0, 0.0 , 0, 'UTC' , - ) ). % Timestamp created with ?- date_time_stamp(date(2000,1,1,0,0,0,0 ,- ,-),Stamp).
stamp_date_time_test_generator(946684800.0 , 'UTC', 0, date(2000, 1, 1, 0, 0, 0.0 , 0, 'UTC' , - ) ). % Timestamp created with ?- date_time_stamp(date(2000,1,1,0,0,0,0 ,local,-),Stamp).
stamp_date_time_test_generator(946684800.0 , 'UTC', 0, date(2000, 1, 1, 0, 0, 0.0 , 0, 'UTC' , - ) ). % Timestamp created with ?- date_time_stamp(date(2000,1,1,0,0,0,0 ,'UTC',-),Stamp).
stamp_date_time_test_generator(946684800.0 , 'UTC', 0, date(2000, 1, 1, 0, 0, 0.0 , 0, 'UTC' , - ) ). % Timestamp created with ?- date_time_stamp(date(2000,1,1,0,0,0,0 ,'Z' ,-),Stamp).
stamp_date_time_test_generator(946702800.0 , 0, 0, date(2000, 1, 1, 5, 0, 0.0 , 0, - , - ) ). % Timestamp created with ?- current_prolog_flag(timezone,Offset),date_time_stamp(date(2000,1,1,0,0,0,Offset ,- ,-),Stamp).
stamp_date_time_test_generator(946702800.0 , 0, 0, date(2000, 1, 1, 5, 0, 0.0 , 0, - , - ) ). % Timestamp created with ?- date_time_stamp(date(2000,1,1,0,0,0,Offset ,local,-),Stamp).
stamp_date_time_test_generator(946702800.0 , 0, 0, date(2000, 1, 1, 5, 0, 0.0 , 0, - , - ) ). % Timestamp created with ?- date_time_stamp(date(2000,1,1,0,0,0,Offset ,'UTC',-),Stamp).
stamp_date_time_test_generator(946702800.0 , 0, 0, date(2000, 1, 1, 5, 0, 0.0 , 0, - , - ) ). % Timestamp created with ?- date_time_stamp(date(2000,1,1,0,0,0,Offset ,'Z' ,-),Stamp).
stamp_date_time_test_generator(946684800.0 , 0, 0, date(2000, 1, 1, 0, 0, 0.0 , 0, - , - ) ). % Timestamp created with ?- date_time_stamp(date(2000,1,1,0,0,0,0 ,- ,-),Stamp).
stamp_date_time_test_generator(946684800.0 , 0, 0, date(2000, 1, 1, 0, 0, 0.0 , 0, - , - ) ). % Timestamp created with ?- date_time_stamp(date(2000,1,1,0,0,0,0 ,local,-),Stamp).
stamp_date_time_test_generator(946684800.0 , 0, 0, date(2000, 1, 1, 0, 0, 0.0 , 0, - , - ) ). % Timestamp created with ?- date_time_stamp(date(2000,1,1,0,0,0,0 ,'UTC',-),Stamp).
stamp_date_time_test_generator(946684800.0 , 0, 0, date(2000, 1, 1, 0, 0, 0.0 , 0, - , - ) ). % Timestamp created with ?- date_time_stamp(date(2000,1,1,0,0,0,0 ,'Z' ,-),Stamp).
test(005,[forall(stamp_date_time_test_generator(Timestamp,Stamp_timezone,Expected_offset,Expected_date_time))]) :-
% format('~nTimestamp: ~w~n',[Timestamp]),
stamp_date_time(Timestamp,Date_time,Stamp_timezone),
% format('Date_time: ~w~n',[Date_time]),
% Date_time = date(Year,Month,Day,Hour,Minute,Seconds,Offset,Timezone_name,Daylight_savings_time),
Date_time = date(_,_,_,_,_,_,Offset,_,_),
(
Expected_offset == current_timezone
->
current_prolog_flag(timezone,Current_timezone),
assertion( Offset == Current_timezone )
;
assertion( Offset == Expected_offset )
),
assertion( Date_time == Expected_date_time ).
:- end_tests(stamp_date_time).
Running of tests.
?- run_tests(stamp_date_time).
% PL-Unit: stamp_date_time ............................. done
% All 29 tests passed
true.