Convert MS-format datetimes without platform-bound fromtimestamp (#59)

tests/test_api_client/test_deserializer.py

@@ -327,6 +327,25 @@ def test_deserialize_datetime_ms_error(data):
         deserialize_datetime_ms(data_type, data, model_finder=None)
 
 
+def test_deserialize_datetime_ms_pre_epoch():
+    # regression for #59: a timestamp before 1970 (negative ms) must convert
+    # cleanly on every platform instead of leaking OSError (datetime.fromtimestamp
+    # rejects negative timestamps on Windows).
+    result = deserialize_datetime_ms(
+        "datetime[ms-format]", "/Date(-2208988800000)/", model_finder=None
+    )
+    assert result == datetime(1900, 1, 1, tzinfo=tz.UTC)
+
+
+def test_deserialize_datetime_ms_out_of_range_raises_value_error():
+    # regression for #59: an out-of-range timestamp must raise the documented
+    # ValueError, not leak OSError / OverflowError from the platform C library.
+    with pytest.raises(ValueError):
+        deserialize_datetime_ms(
+            "datetime[ms-format]", "/Date(67768036191676800000)/", model_finder=None
+        )
+
+
 # deserialize_model tests
 def test_deserialize_model():
     # given test model and test data

xero_python/api_client/deserializer.py

@@ -250,8 +250,17 @@ def deserialize_datetime_ms(data_type, data, model_finder):
             tz_info = tz.UTC
 
         timestamp_ms = int(match.groupdict()["timestamp"])
-        timestamp_s = timestamp_ms / 1000
-        return datetime.datetime.fromtimestamp(timestamp_s, tz=tz_info)
+        # datetime.fromtimestamp is bounded by the platform's C library and
+        # leaks OSError/OverflowError for out-of-range or (on Windows)
+        # pre-1970 timestamps. Build the datetime from the epoch instead so
+        # the conversion is platform independent, and surface the documented
+        # ValueError for values python's datetime cannot represent.
+        epoch = datetime.datetime(1970, 1, 1, tzinfo=tz.UTC)
+        try:
+            utc_datetime = epoch + datetime.timedelta(milliseconds=timestamp_ms)
+        except (OverflowError, OSError) as error:
+            raise ValueError("Invalid datetime value {!r}".format(data)) from error
+        return utc_datetime.astimezone(tz_info)
     elif DATE_WITH_NO_DAY_RE.match(str(data)):
         return datetime.datetime.strptime(data + "-01", "%Y-%m-%d")
     else: