Work with Windows’ different epoch (#98)

Fixes #89.

HISTORY.rst

@@ -11,6 +11,7 @@ History
 * Remove ``tz_offset`` argument. This was incorrectly copied from
   ``freezegun``. Use the new timezone mocking with ``ZoneInfo`` instead.
 * Add pytest plugin and fixture ``time_machine``.
+* Work with Windows’ different epoch.
 
 1.3.0 (2020-12-12)
 ------------------

src/time_machine.py

@@ -4,6 +4,7 @@
 import os
 import sys
 import uuid
+from time import gmtime as orig_gmtime
 from time import tzset
 from types import GeneratorType
 from typing import Optional
@@ -33,6 +34,22 @@
 
 NANOSECONDS_PER_SECOND = 1_000_000_000
 
+# Windows' time epoch is not unix epoch but in 1601. This constant helps us
+# translate to it.
+_system_epoch = orig_gmtime(0)
+SYSTEM_EPOCH_TIMESTAMP_NS = int(
+    dt.datetime(
+        _system_epoch.tm_year,
+        _system_epoch.tm_mon,
+        _system_epoch.tm_mday,
+        _system_epoch.tm_hour,
+        _system_epoch.tm_min,
+        _system_epoch.tm_sec,
+        tzinfo=dt.timezone.utc,
+    ).timestamp()
+    * NANOSECONDS_PER_SECOND
+)
+
 
 def extract_timestamp_tzname(destination):
     if callable(destination):
@@ -82,14 +99,15 @@ def time_ns(self):
         if not self._tick:
             return self._destination_timestamp_ns
 
+        base = SYSTEM_EPOCH_TIMESTAMP_NS + self._destination_timestamp_ns
         now_ns = self._time_ns()
 
         if not self._requested:
             self._requested = True
             self._real_start_timestamp_ns = now_ns
-            return self._destination_timestamp_ns
+            return base
 
-        return self._destination_timestamp_ns + (now_ns - self._real_start_timestamp_ns)
+        return base + (now_ns - self._real_start_timestamp_ns)
 
     if sys.version_info >= (3, 7):
 

tests/test_time_machine.py

@@ -4,7 +4,7 @@
 import time
 import uuid
 from importlib.util import module_from_spec, spec_from_file_location
-from unittest import SkipTest, TestCase
+from unittest import SkipTest, TestCase, mock
 
 import pytest
 from dateutil import tz
@@ -249,6 +249,25 @@ def test_time_time():
     assert now >= LIBRARY_EPOCH
 
 
+windows_epoch_in_posix = -11_644_445_222
+
+
+@mock.patch.object(
+    time_machine,
+    "SYSTEM_EPOCH_TIMESTAMP_NS",
+    (windows_epoch_in_posix * NANOSECONDS_PER_SECOND),
+)
+def test_time_time_windows():
+    with time_machine.travel(EPOCH):
+        first = time.time()
+        assert isinstance(first, float)
+        assert first == windows_epoch_in_posix
+
+        second = time.time()
+        assert isinstance(second, float)
+        assert windows_epoch_in_posix < second < windows_epoch_in_posix + 1.0
+
+
 def test_time_time_no_tick():
     with time_machine.travel(EPOCH, tick=False):
         assert time.time() == EPOCH