handle variable timezone offsets in dates due daylight saving

rebar.config

@@ -30,7 +30,8 @@
                                 inline,
                                 {platform_define, "^R[01][0-9]", 'NO_MAP_TYPE'},
                                 {platform_define, "^(R|17)", 'NO_DIALYZER_SPEC'},
-                                {d, 'MAP_ITER_ORDERED'}]}]},
+                                {d, 'MAP_ITER_ORDERED'},
+                                {d, 'TIME_MODULE', test_time_module}]}]},
             {edown, [{edoc_opts, [{doclet, edown_doclet}]}, {deps, [edown]}]}]}.
 
 {project_plugins, [covertool, rebar3_efmt]}.

src/jsone.erl

@@ -176,6 +176,8 @@
 %% > jsone:encode({{2000, 3, 10}, {10, 3, 58}}, [{datetime_format, {iso8601, local}}]).
 %% <<"\"2000-03-10T10:03:58+09:00\"">>
 %%
+%% % Also you can use {iso8601, local_dst} to properly calculate the timezone according to the daylight saving procedure. Consider using it, if the executing computer is located in a country that implements this procedure
+%%
 %% %
 %% % Explicit TimeZone Offset
 %% %
@@ -184,7 +186,7 @@
 %% '''
 
 -type datetime_format() :: iso8601.
--type timezone() :: utc | local | utc_offset_seconds().
+-type timezone() :: utc | local | local_dst | utc_offset_seconds().
 -type utc_offset_seconds() :: -86399..86399.
 
 -type common_option() :: undefined_as_null.

src/jsone_encode.erl

@@ -580,6 +580,8 @@ parse_option([{datetime_format, Fmt} | T], Opt) ->
             parse_option(T, Opt?OPT{datetime_format = {iso8601, 0}});
         {iso8601, local} ->
             parse_option(T, Opt?OPT{datetime_format = {iso8601, local_offset()}});
+        {iso8601, local_dst} ->
+            parse_option(T, Opt?OPT{datetime_format = {iso8601, local_offset_dst()}});
         {iso8601, N} when -86400 < N, N < 86400 ->
             parse_option(T, Opt?OPT{datetime_format = {iso8601, N}});
         _ ->
@@ -600,3 +602,17 @@ local_offset() ->
     UTC = {{1970, 1, 2}, {0, 0, 0}},
     Local = calendar:universal_time_to_local_time({{1970, 1, 2}, {0, 0, 0}}),
     calendar:datetime_to_gregorian_seconds(Local) - calendar:datetime_to_gregorian_seconds(UTC).
+
+
+-ifndef(TIME_MODULE).
+
+-define(TIME_MODULE, erlang).
+
+-endif.
+
+
+-spec local_offset_dst() -> jsone:utc_offset_seconds().
+local_offset_dst() ->
+    LocalDateTime = ?TIME_MODULE:localtime(),
+    calendar:datetime_to_gregorian_seconds(LocalDateTime) -
+    calendar:datetime_to_gregorian_seconds(?TIME_MODULE:localtime_to_universaltime(LocalDateTime)).

test/jsone_decode_tests.erl

@@ -43,7 +43,8 @@ decode_test_() ->
               ?assertEqual({ok, 0, <<"1">>}, jsone_decode:decode(<<"-01">>))
       end},
      {"integer can't begin with an explicit plus sign",
-      fun() -> ?assertMatch({error, {badarg, _}}, jsone_decode:decode(<<"+1">>)) end},
+      fun() -> ?assertMatch({error, {badarg, _}}, jsone_decode:decode(<<"+1">>))
+      end},
 
      %% Numbers: Floats
      {"float: decimal notation", fun() -> ?assertEqual({ok, 1.23, <<"">>}, jsone_decode:decode(<<"1.23">>)) end},

test/jsone_encode_tests.erl

@@ -140,6 +140,22 @@ encode_test_() ->
                       ?assertMatch(<<"\"2015-06-25T14:57:25Z\"">>, Json)
               end
       end},
+     {"datetime: iso8601: local with daylight saving variable zone - summer time (2h offset)",
+      fun() ->
+              test_time_module:set_localtime({{2024, 9, 15}, {11, 00, 00}}),
+              test_time_module:mock_localtime_to_universaltime(fun(_) -> {{2024, 9, 15}, {9, 00, 00}} end),
+
+              {ok, Json} = jsone_encode:encode({{2015, 6, 25}, {14, 57, 25}}, [{datetime_format, {iso8601, local_dst}}]),
+              ?assertMatch(<<"\"2015-06-25T14:57:25+02:00\"">>, Json)
+      end},
+     {"datetime: iso8601: local with daylight saving variable zone - winter time (1h offset)",
+      fun() ->
+              test_time_module:set_localtime({{2024, 12, 15}, {11, 00, 00}}),
+              test_time_module:mock_localtime_to_universaltime(fun(_) -> {{2024, 12, 15}, {10, 00, 00}} end),
+
+              {ok, Json} = jsone_encode:encode({{2015, 6, 25}, {14, 57, 25}}, [{datetime_format, {iso8601, local_dst}}]),
+              ?assertMatch(<<"\"2015-06-25T14:57:25+01:00\"">>, Json)
+      end},
      {"datetime: iso8601: timezone",
       fun() ->
               ?assertEqual({ok, <<"\"2015-06-25T14:57:25Z\"">>},

test/test_time_module.erl

@@ -0,0 +1,19 @@
+-module(test_time_module).
+-export([localtime/0, set_localtime/1, localtime_to_universaltime/1, mock_localtime_to_universaltime/1]).
+
+
+set_localtime({{_, _, _}, {_, _, _}} = LocalTime) ->
+    erlang:put('__test_time_module__localtime__', LocalTime).
+
+
+localtime() ->
+    erlang:get('__test_time_module__localtime__').
+
+
+localtime_to_universaltime({{_, _, _}, {_, _, _}} = LocalTime) ->
+    LocalTimeToUniversalTimeFun = erlang:get('__test_time_module_localtime_to_universaltime__'),
+    LocalTimeToUniversalTimeFun(LocalTime).
+
+
+mock_localtime_to_universaltime(Fun) when is_function(Fun) ->
+    erlang:put('__test_time_module_localtime_to_universaltime__', Fun).