Merge branch 'hasse/stdlib/rfc3339_datetime/OTP-14764'

* hasse/stdlib/rfc3339_datetime/OTP-14764:
  stdlib: Add RFC 3339 functions to module calendar

3 files changed, 369 insertions, 8 deletions

diff --git a/lib/stdlib/doc/src/calendar.xml b/lib/stdlib/doc/src/calendar.xml
index 0c4a30ce16..8f2b6b747a 100644
--- a/lib/stdlib/doc/src/calendar.xml
+++ b/lib/stdlib/doc/src/calendar.xml
@@ -317,6 +317,30 @@
     </func>
 
     <func>
+      <name name="rfc3339_to_system_time" arity="1"/>
+      <name name="rfc3339_to_system_time" arity="2"/>
+      <fsummary>Convert from RFC 3339 timestamp to system time.</fsummary>
+      <type name="rfc3339_string"/>
+      <type name="rfc3339_time_unit"/>
+      <desc>
+	<p>Converts an RFC 3339 timestamp into system time.</p>
+	<p>Valid option:</p>
+	<taglist>
+	  <tag><c>{unit, Unit}</c></tag>
+	  <item><p>The time unit of the return value.
+	    The default is <c>second</c>.</p>
+	  </item>
+	</taglist>
+        <pre>
+1> <input>calendar:rfc3339_to_system_time("2018-02-01T16:17:58+01:00").</input>
+1517498278
+2> <input>calendar:rfc3339_to_system_time("2018-02-01 15:18:02.088Z",
+   [{unit, nanosecond}]).</input>
+1517498282088000000</pre>
+      </desc>
+    </func>
+
+    <func>
       <name name="seconds_to_daystime" arity="1"/>
       <fsummary>Compute days and time from seconds.</fsummary>
       <desc>
@@ -347,6 +371,52 @@
     </func>
 
     <func>
+      <name name="system_time_to_rfc3339" arity="1"/>
+      <name name="system_time_to_rfc3339" arity="2"/>
+      <fsummary>Convert from system to RFC 3339 timestamp.</fsummary>
+      <type name="offset"/>
+      <type name="rfc3339_string"/>
+      <type name="rfc3339_time_unit"/>
+      <desc>
+	<p>Converts a system time into RFC 3339 timestamp.</p>
+	<p>Valid options:</p>
+	<taglist>
+	  <tag><c>{offset, Offset}</c></tag>
+	  <item><p>The offset, either a string or an integer, to be
+	    included in the formatted string.
+	    An empty string, which is the default, is interpreted
+	    as local time. A non-empty string is included as is.
+	    The time unit of the integer is the same as the one
+	    of <c><anno>Time</anno></c>.</p>
+	  </item>
+	  <tag><c>{time_designator, Character}</c></tag>
+	  <item><p>The character used as time designator, that is,
+	    the date and time separator. The default is <c>$T</c>.</p>
+	  </item>
+	  <tag><c>{unit, Unit}</c></tag>
+	  <item><p>The time unit of <c><anno>Time</anno></c>. The
+	    default is <c>second</c>. If some other unit is given
+	    (<c>millisecond</c>, <c>microsecond</c>, or
+	    <c>nanosecond</c>), the formatted string includes a
+	    fraction of a second.</p>
+	  </item>
+	</taglist>
+        <pre>
+1> <input>calendar:system_time_to_rfc3339(erlang:system_time(second)).</input>
+"2018-04-23T14:56:28+02:00"
+2> <input>calendar:system_time_to_rfc3339(erlang:system_time(second),
+   [{offset, "-02:00"}]).</input>
+"2018-04-23T10:56:52-02:00"
+3> <input>calendar:system_time_to_rfc3339(erlang:system_time(second),
+   [{offset, -7200}]).</input>
+"2018-04-23T10:57:05-02:00"
+4> <input>calendar:system_time_to_rfc3339(erlang:system_time(millisecond),
+   [{unit, millisecond}, {time_designator, $\s}, {offset, "Z"}]).</input>
+"2018-04-23 12:57:20.482Z"</pre>
+      </desc>
+    </func>
+
+    <func>
       <name name="system_time_to_universal_time" arity="2"/>
       <fsummary>Convert system time to universal date and time.</fsummary>
       <desc>
diff --git a/lib/stdlib/src/calendar.erl b/lib/stdlib/src/calendar.erl
index 2e24e8c133..9a600c1972 100644
--- a/lib/stdlib/src/calendar.erl
+++ b/lib/stdlib/src/calendar.erl
@@ -39,10 +39,14 @@
 	 now_to_datetime/1,			% = now_to_universal_time/1
 	 now_to_local_time/1,
 	 now_to_universal_time/1,
+         rfc3339_to_system_time/1,
+         rfc3339_to_system_time/2,
 	 seconds_to_daystime/1,
 	 seconds_to_time/1,
          system_time_to_local_time/2,
          system_time_to_universal_time/2,
+         system_time_to_rfc3339/1,
+         system_time_to_rfc3339/2,
 	 time_difference/2,
 	 time_to_seconds/1,
 	 universal_time/0,
@@ -57,11 +61,13 @@
 -define(SECONDS_PER_DAY, 86400).
 -define(DAYS_PER_YEAR, 365).
 -define(DAYS_PER_LEAP_YEAR, 366).
--define(DAYS_PER_4YEARS, 1461).
--define(DAYS_PER_100YEARS, 36524).
--define(DAYS_PER_400YEARS, 146097).
+%% -define(DAYS_PER_4YEARS, 1461).
+%% -define(DAYS_PER_100YEARS, 36524).
+%% -define(DAYS_PER_400YEARS, 146097).
 -define(DAYS_FROM_0_TO_1970, 719528).
+-define(DAYS_FROM_0_TO_10000, 2932897).
 -define(SECONDS_FROM_0_TO_1970, (?DAYS_FROM_0_TO_1970*?SECONDS_PER_DAY)).
+-define(SECONDS_FROM_0_TO_10000, (?DAYS_FROM_0_TO_10000*?SECONDS_PER_DAY)).
 
 %%----------------------------------------------------------------------
 %% Types
@@ -86,6 +92,13 @@
 -type datetime1970()   :: {{year1970(),month(),day()},time()}.
 -type yearweeknum()    :: {year(),weeknum()}.
 
+-type rfc3339_string() :: [byte(), ...].
+%% By design 'native' is not supported:
+-type rfc3339_time_unit() :: 'microsecond'
+                           | 'millisecond'
+                           | 'nanosecond'
+                           | 'second'.
+
 %%----------------------------------------------------------------------
 
 %% All dates are according the the Gregorian calendar. In this module
@@ -312,8 +325,7 @@ local_time_to_universal_time_dst(DateTime) ->
 -spec now_to_datetime(Now) -> datetime1970() when
       Now :: erlang:timestamp().
 now_to_datetime({MSec, Sec, _uSec}) ->
-    Sec0 = MSec*1000000 + Sec + ?SECONDS_FROM_0_TO_1970,
-    gregorian_seconds_to_datetime(Sec0).
+    system_time_to_datetime(MSec*1000000 + Sec).
 
 -spec now_to_universal_time(Now) -> datetime1970() when
       Now :: erlang:timestamp().
@@ -331,6 +343,33 @@ now_to_local_time({MSec, Sec, _uSec}) ->
     erlang:universaltime_to_localtime(
       now_to_universal_time({MSec, Sec, _uSec})).
 
+-spec rfc3339_to_system_time(DateTimeString) -> integer() when
+      DateTimeString :: rfc3339_string().
+
+rfc3339_to_system_time(DateTimeString) ->
+    rfc3339_to_system_time(DateTimeString, []).
+
+-spec rfc3339_to_system_time(DateTimeString, Options) -> integer() when
+      DateTimeString :: rfc3339_string(),
+      Options :: [Option],
+      Option :: {'unit', rfc3339_time_unit()}.
+
+rfc3339_to_system_time(DateTimeString, Options) ->
+    Unit = proplists:get_value(unit, Options, second),
+    %% _T is the character separating the date and the time:
+    {DateStr, [_T|TimeStr]} = lists:split(10, DateTimeString),
+    {TimeStr2, TimeStr3} = lists:split(8, TimeStr),
+    {ok, [Hour, Min, Sec], []} = io_lib:fread("~d:~d:~d", TimeStr2),
+    {ok, [Year, Month, Day], []} = io_lib:fread("~d-~d-~d", DateStr),
+    DateTime = {{Year, Month, Day}, {Hour, Min, Sec}},
+    IsFractionChar = fun(C) -> C >= $0 andalso C =< $9 orelse C =:= $. end,
+    {FractionStr, UtcOffset} = lists:splitwith(IsFractionChar, TimeStr3),
+    Time = datetime_to_system_time(DateTime),
+    Secs = Time - offset_adjustment(Time, second, UtcOffset),
+    check(DateTimeString, Options, Secs),
+    ScaledEpoch = erlang:convert_time_unit(Secs, second, Unit),
+    ScaledEpoch + copy_sign(fraction(Unit, FractionStr), ScaledEpoch).
+
 
 
 %% seconds_to_daystime(Secs) = {Days, {Hour, Minute, Second}}
@@ -380,7 +419,40 @@ system_time_to_local_time(Time, TimeUnit) ->
 
 system_time_to_universal_time(Time, TimeUnit) ->
     Secs = erlang:convert_time_unit(Time, TimeUnit, second),
-    gregorian_seconds_to_datetime(Secs + ?SECONDS_FROM_0_TO_1970).
+    system_time_to_datetime(Secs).
+
+-spec system_time_to_rfc3339(Time) -> DateTimeString when
+      Time :: integer(),
+      DateTimeString :: rfc3339_string().
+
+system_time_to_rfc3339(Time) ->
+    system_time_to_rfc3339(Time, []).
+
+-type offset() :: [byte()] | (Time :: integer()).
+-spec system_time_to_rfc3339(Time, Options) -> DateTimeString when
+      Time :: integer(), % Since Epoch
+      Options :: [Option],
+      Option :: {'offset', offset()}
+              | {'time_designator', byte()}
+              | {'unit', rfc3339_time_unit()},
+      DateTimeString :: rfc3339_string().
+
+system_time_to_rfc3339(Time, Options) ->
+    Unit = proplists:get_value(unit, Options, second),
+    OffsetOption = proplists:get_value(offset, Options, ""),
+    T = proplists:get_value(time_designator, Options, $T),
+    AdjustmentSecs = offset_adjustment(Time, Unit, OffsetOption),
+    Offset = offset(OffsetOption, AdjustmentSecs),
+    Adjustment = erlang:convert_time_unit(AdjustmentSecs, second, Unit),
+    AdjustedTime = Time + Adjustment,
+    Factor = factor(Unit),
+    Secs = AdjustedTime div Factor,
+    check(Time, Options, Secs),
+    DateTime = system_time_to_datetime(Secs),
+    {{Year, Month, Day}, {Hour, Min, Sec}} = DateTime,
+    FractionStr = fraction_str(Factor, AdjustedTime),
+    flat_fwrite("~4.10.0B-~2.10.0B-~2.10.0B~c~2.10.0B:~2.10.0B:~2.10.0B~s~s",
+                [Year, Month, Day, T, Hour, Min, Sec, FractionStr, Offset]).
 
 %% time_difference(T1, T2) = Tdiff
 %%
@@ -569,3 +641,85 @@ df(Year, _) ->
 	true -> 1;
 	false  -> 0
     end.
+
+check(_Arg, _Options, Secs) when Secs >= - ?SECONDS_FROM_0_TO_1970,
+                                 Secs < ?SECONDS_FROM_0_TO_10000 ->
+    ok;
+check(Arg, Options, _Secs) ->
+    erlang:error({badarg, [Arg, Options]}).
+
+datetime_to_system_time(DateTime) ->
+    datetime_to_gregorian_seconds(DateTime) - ?SECONDS_FROM_0_TO_1970.
+
+system_time_to_datetime(Seconds) ->
+    gregorian_seconds_to_datetime(Seconds + ?SECONDS_FROM_0_TO_1970).
+
+offset(OffsetOption, Secs0) when OffsetOption =:= "";
+                                 is_integer(OffsetOption) ->
+    Sign = case Secs0 < 0 of
+               true -> $-;
+               false -> $+
+           end,
+    Secs = abs(Secs0),
+    Hour = Secs div 3600,
+    Min = (Secs rem 3600) div 60,
+    io_lib:fwrite("~c~2.10.0B:~2.10.0B", [Sign, Hour, Min]);
+offset(OffsetOption, _Secs) ->
+    OffsetOption.
+
+offset_adjustment(Time, Unit, OffsetString) when is_list(OffsetString) ->
+    offset_string_adjustment(Time, Unit, OffsetString);
+offset_adjustment(_Time, Unit, Offset) when is_integer(Offset) ->
+    erlang:convert_time_unit(Offset, Unit, second).
+
+offset_string_adjustment(Time, Unit, "") ->
+    local_offset(Time, Unit);
+offset_string_adjustment(_Time, _Unit, "Z") ->
+    0;
+offset_string_adjustment(_Time, _Unit, "z") ->
+    0;
+offset_string_adjustment(_Time, _Unit, [Sign|Tz]) ->
+    {ok, [Hour, Min], []} = io_lib:fread("~d:~d", Tz),
+    Adjustment = 3600 * Hour + 60 * Min,
+    case Sign of
+        $- -> -Adjustment;
+        $+ -> Adjustment
+    end.
+
+local_offset(SystemTime, Unit) ->
+    LocalTime = system_time_to_local_time(SystemTime, Unit),
+    UniversalTime = system_time_to_universal_time(SystemTime, Unit),
+    LocalSecs = datetime_to_gregorian_seconds(LocalTime),
+    UniversalSecs = datetime_to_gregorian_seconds(UniversalTime),
+    LocalSecs - UniversalSecs.
+
+fraction_str(Factor, Time) ->
+    case Time rem Factor of
+        0 ->
+            "";
+        Fraction ->
+            FS = io_lib:fwrite(".~*..0B", [log10(Factor), abs(Fraction)]),
+            string:trim(FS, trailing, "0")
+    end.
+
+fraction(second, _) ->
+    0;
+fraction(_, "") ->
+    0;
+fraction(Unit, FractionStr) ->
+    round(factor(Unit) * list_to_float([$0|FractionStr])).
+
+copy_sign(N1, N2) when N2 < 0 -> -N1;
+copy_sign(N1, _N2) -> N1.
+
+factor(second)      -> 1;
+factor(millisecond) -> 1000;
+factor(microsecond) -> 1000000;
+factor(nanosecond)  -> 1000000000.
+
+log10(1000) -> 3;
+log10(1000000) -> 6;
+log10(1000000000) -> 9.
+
+flat_fwrite(F, S) ->
+    lists:flatten(io_lib:fwrite(F, S)).
diff --git a/lib/stdlib/test/calendar_SUITE.erl b/lib/stdlib/test/calendar_SUITE.erl
index 52c3cc68eb..55118e251c 100644
--- a/lib/stdlib/test/calendar_SUITE.erl
+++ b/lib/stdlib/test/calendar_SUITE.erl
@@ -31,7 +31,7 @@
 	 last_day_of_the_month/1,
 	 local_time_to_universal_time_dst/1,
 	 iso_week_number/1,
-         system_time/1]).
+         system_time/1, rfc3339/1]).
 
 -define(START_YEAR, 1947).			
 -define(END_YEAR, 2012).
@@ -42,7 +42,7 @@ all() ->
     [gregorian_days, gregorian_seconds, day_of_the_week,
      day_of_the_week_calibrate, leap_years,
      last_day_of_the_month, local_time_to_universal_time_dst,
-     iso_week_number, system_time].
+     iso_week_number, system_time, rfc3339].
 
 groups() -> 
     [].
@@ -175,10 +175,147 @@ system_time(Config) when is_list(Config) ->
 
     ok.
 
+rfc3339(Config) when is_list(Config) ->
+    Ms = [{unit, millisecond}],
+    Mys = [{unit, microsecond}],
+    Ns = [{unit, nanosecond}],
+    S = [{unit, second}],
+    D = [{time_designator, $\s}],
+    Z = [{offset, "Z"}],
+
+    "1985-04-12T23:20:50.52Z" = test_parse("1985-04-12T23:20:50.52Z", Ms),
+    "1985-04-12T23:20:50.52Z" = test_parse("1985-04-12t23:20:50.52z", Ms),
+    "1985-04-12T21:20:50.52Z" =
+        test_parse("1985-04-12T23:20:50.52+02:00", Ms),
+    "1985-04-12T23:20:50Z" = test_parse("1985-04-12T23:20:50.52Z", S),
+    "1985-04-12T23:20:50.52Z" = test_parse("1985-04-12T23:20:50.52Z", Ms),
+    "1985-04-12T23:20:50.52Z" = test_parse("1985-04-12t23:20:50.52z", Mys),
+    "1985-04-12 21:20:50.52Z" =
+        test_parse("1985-04-12 23:20:50.52+02:00", Ns++D),
+    "1985-04-12T23:20:50Z" = test_parse("1985-04-12T23:20:50.52Z"),
+    "1996-12-20T00:39:57Z" = test_parse("1996-12-19T16:39:57-08:00"),
+    "1991-01-01T00:00:00Z" = test_parse("1990-12-31T23:59:60Z"),
+    "1991-01-01T08:00:00Z" = test_parse("1990-12-31T23:59:60-08:00"),
+
+    "1996-12-20T00:39:57Z" = test_parse("1996-12-19T16:39:57-08:00"),
+    %% The leap second is not handled:
+    "1991-01-01T00:00:00Z" = test_parse("1990-12-31T23:59:60Z"),
+
+    "9999-12-31T23:59:59Z" = do_format_z(253402300799, []),
+    "9999-12-31T23:59:59.999Z" = do_format_z(253402300799*1000+999, Ms),
+    "9999-12-31T23:59:59.999999Z" =
+        do_format_z(253402300799*1000000+999999, Mys),
+    "9999-12-31T23:59:59.999999999Z" =
+        do_format_z(253402300799*1000000000+999999999, Ns),
+    {'EXIT', _} = (catch do_format_z(253402300799+1, [])),
+    {'EXIT', _} = (catch do_parse("9999-12-31T23:59:60Z", [])),
+    {'EXIT', _} = (catch do_format_z(253402300799*1000000000+999999999+1, Ns)),
+    253402300799 = do_parse("9999-12-31T23:59:59Z", []),
+
+    "0000-01-01T00:00:00Z" = test_parse("0000-01-01T00:00:00.0+00:00"),
+    "9999-12-31T00:00:00Z" = test_parse("9999-12-31T00:00:00.0+00:00"),
+    "1584-03-04T00:00:00Z" = test_parse("1584-03-04T00:00:00.0+00:00"),
+    "1900-01-01T00:00:00Z" = test_parse("1900-01-01T00:00:00.0+00:00"),
+    "2016-01-24T00:00:00Z" = test_parse("2016-01-24T00:00:00.0+00:00"),
+    "1970-01-01T00:00:00Z" = test_parse("1970-01-01T00:00:00Z"),
+    "1970-01-02T00:00:00Z" = test_parse("1970-01-01T23:59:60Z"),
+    "1970-01-02T00:00:00Z" = test_parse("1970-01-01T23:59:60.5Z"),
+    "1970-01-02T00:00:00Z" = test_parse("1970-01-01T23:59:60.55Z"),
+    "1970-01-02T00:00:00.55Z" = test_parse("1970-01-01T23:59:60.55Z", Ms),
+    "1970-01-02T00:00:00.55Z" = test_parse("1970-01-01T23:59:60.55Z", Mys),
+    "1970-01-02T00:00:00.55Z" = test_parse("1970-01-01T23:59:60.55Z", Ns),
+    "1970-01-02T00:00:00.999999Z" =
+        test_parse("1970-01-01T23:59:60.999999Z", Mys),
+    "1970-01-02T00:00:01Z" =
+        test_parse("1970-01-01T23:59:60.999999Z", Ms),
+    "1970-01-01T00:00:00Z" = test_parse("1970-01-01T00:00:00+00:00"),
+    "1970-01-01T00:00:00Z" = test_parse("1970-01-01T00:00:00-00:00"),
+    "1969-12-31T00:01:00Z" = test_parse("1970-01-01T00:00:00+23:59"),
+    "1918-11-11T09:00:00Z" = test_parse("1918-11-11T11:00:00+02:00", Mys),
+    "1970-01-01T00:00:00.000001Z" =
+        test_parse("1970-01-01T00:00:00.000001Z", Mys),
+
+    test_time(erlang:system_time(second), []),
+    test_time(erlang:system_time(second), Z),
+    test_time(erlang:system_time(second), Z ++ S),
+    test_time(erlang:system_time(second), [{offset, "+02:20"}]),
+    test_time(erlang:system_time(millisecond), Ms),
+    test_time(erlang:system_time(microsecond), Mys++[{offset, "-02:20"}]),
+
+    T = erlang:system_time(second),
+    TS = do_format(T, []),
+    TS = do_format(T * 1000, Ms),
+    TS = do_format(T * 1000 * 1000, Mys),
+    TS = do_format(T * 1000 * 1000 * 1000, Ns),
+
+    946720800 = TO = do_parse("2000-01-01 10:00:00Z", []),
+    Str = "2000-01-01T10:02:00+00:02",
+    Str = do_format(TO, [{offset, 120}]),
+    Str = do_format(TO * 1000, [{offset, 120 * 1000}]++Ms),
+    Str = do_format(TO * 1000 * 1000, [{offset, 120 * 1000 * 1000}]++Mys),
+    Str = do_format(TO * 1000 * 1000 * 1000,
+                    [{offset, 120 * 1000 * 1000 * 1000}]++Ns),
+
+    NStr = "2000-01-01T09:58:00-00:02",
+    NStr = do_format(TO, [{offset, -120}]),
+    NStr = do_format(TO * 1000, [{offset, -120 * 1000}]++Ms),
+    NStr = do_format(TO * 1000 * 1000, [{offset, -120 * 1000 * 1000}]++Mys),
+    NStr = do_format(TO * 1000 * 1000 * 1000,
+                     [{offset, -120 * 1000 * 1000 * 1000}]++Ns),
+
+    543210000 = do_parse("1970-01-01T00:00:00.54321Z", Ns),
+    54321000 = do_parse("1970-01-01T00:00:00.054321Z", Ns),
+    543210 = do_parse("1970-01-01T00:00:00.54321Z", Mys),
+    543 = do_parse("1970-01-01T00:00:00.54321Z", Ms),
+    0 = do_parse("1970-01-01T00:00:00.000001Z", Ms),
+    1 = do_parse("1970-01-01T00:00:00.000001Z", Mys),
+    1000 = do_parse("1970-01-01T00:00:00.000001Z", Ns),
+    0 = do_parse("1970-01-01Q00:00:00.00049Z", Ms),
+    1 = do_parse("1970-01-01Q00:00:00.0005Z", Ms),
+    6543210 = do_parse("1970-01-01T00:00:06.54321Z", Mys),
+    298815132000000 = do_parse("1979-06-21T12:12:12Z", Mys),
+    -1613826000000000 = do_parse("1918-11-11T11:00:00Z", Mys),
+    -1613833200000000 = do_parse("1918-11-11T11:00:00+02:00", Mys),
+    -1613833200000000 = do_parse("1918-11-11T09:00:00Z", Mys),
+
+    "1970-01-01T00:00:00Z" = do_format_z(0, Mys),
+    "1970-01-01T00:00:01Z" = do_format_z(1, S),
+    "1970-01-01T00:00:00.001Z" = do_format_z(1, Ms),
+    "1970-01-01T00:00:00.000001Z" = do_format_z(1, Mys),
+    "1970-01-01T00:00:00.000000001Z" = do_format_z(1, Ns),
+    "1970-01-01T00:00:01Z" = do_format_z(1000000, Mys),
+    "1970-01-01T00:00:00.54321Z" = do_format_z(543210, Mys),
+    "1970-01-01T00:00:00.543Z" = do_format_z(543, Ms),
+    "1970-01-01T00:00:00.54321Z" = do_format_z(543210000, Ns),
+    "1970-01-01T00:00:06.54321Z" = do_format_z(6543210, Mys),
+    "1979-06-21T12:12:12Z" = do_format_z(298815132000000, Mys),
+    "1918-11-11T13:00:00Z" = do_format_z(-1613818800000000, Mys),
+    ok.
+
 %%
 %% LOCAL FUNCTIONS
 %%
 
+test_parse(String) ->
+    test_parse(String, []).
+
+test_parse(String, Options) ->
+    T = do_parse(String, Options),
+    calendar:system_time_to_rfc3339(T, [{offset, "Z"} | Options]).
+
+do_parse(String, Options) ->
+    calendar:rfc3339_to_system_time(String, Options).
+
+test_time(Time, Options) ->
+    F = calendar:system_time_to_rfc3339(Time, Options),
+    Time = calendar:rfc3339_to_system_time(F, Options).
+
+do_format_z(Time, Options) ->
+    do_format(Time, [{offset, "Z"}|Options]).
+
+do_format(Time, Options) ->
+    calendar:system_time_to_rfc3339(Time, Options).
+
 %% check_gregorian_days
 %% 
 check_gregorian_days(Days, MaxDays) when Days < MaxDays ->