Fix (de)serialization of negative timestamps (#1241)

telethon/tl/tlobject.py

@@ -1,7 +1,21 @@
 import base64
 import json
 import struct
-from datetime import datetime, date, timedelta
+from datetime import datetime, date, timedelta, timezone
+import time
+
+_EPOCH_NAIVE = datetime(*time.gmtime(0)[:6])
+_EPOCH_NAIVE_LOCAL = datetime(*time.localtime(0)[:6])
+_EPOCH = _EPOCH_NAIVE.replace(tzinfo=timezone.utc)
+
+
+def _datetime_to_timestamp(dt):
+    # If no timezone is specified, it is assumed to be in utc zone
+    if dt.tzinfo is None:
+        dt = dt.replace(tzinfo=timezone.utc)
+    # We use .total_seconds() method instead of simply dt.timestamp(), 
+    # because on Windows the latter raises OSError on datetimes ~< datetime(1970,1,1)
+    return int((dt - _EPOCH).total_seconds())
 
 
 def _json_default(value):
@@ -121,21 +135,21 @@ class TLObject:
 
     @staticmethod
     def serialize_datetime(dt):
-        if not dt:
+        if not dt and not isinstance(dt, timedelta):
             return b'\0\0\0\0'
 
         if isinstance(dt, datetime):
-            dt = int(dt.timestamp())
+            dt = _datetime_to_timestamp(dt)
         elif isinstance(dt, date):
-            dt = int(datetime(dt.year, dt.month, dt.day).timestamp())
+            dt = _datetime_to_timestamp(datetime(dt.year, dt.month, dt.day))
         elif isinstance(dt, float):
             dt = int(dt)
         elif isinstance(dt, timedelta):
-            # Timezones are tricky. datetime.now() + ... timestamp() works
-            dt = int((datetime.now() + dt).timestamp())
+            # Timezones are tricky. datetime.utcnow() + ... timestamp() works
+            dt = _datetime_to_timestamp(datetime.utcnow() + dt)
 
         if isinstance(dt, int):
-            return struct.pack('<I', dt)
+            return struct.pack('<i', dt)
 
         raise TypeError('Cannot interpret "{}" as a date.'.format(dt))