1 files changed, 34 insertions, 30 deletions

diff --git a/Lib/datetime.py b/Lib/datetime.py
index db13b12ef3..6b2ac33a32 100644
--- a/Lib/datetime.py
+++ b/Lib/datetime.py
@@ -316,6 +316,14 @@ def _divide_and_round(a, b):
 
     return q
 
+def _round_half_up(x):
+    """Round to nearest with ties going away from zero."""
+    if x >= 0.0:
+        return _math.floor(x + 0.5)
+    else:
+        return _math.ceil(x - 0.5)
+
+
 class timedelta:
     """Represent the difference between two datetime objects.
 
@@ -399,7 +407,7 @@ class timedelta:
         # secondsfrac isn't referenced again
 
         if isinstance(microseconds, float):
-            microseconds = round(microseconds + usdouble)
+            microseconds = _round_half_up(microseconds + usdouble)
             seconds, microseconds = divmod(microseconds, 1000000)
             days, seconds = divmod(seconds, 24*3600)
             d += days
@@ -410,7 +418,7 @@ class timedelta:
             days, seconds = divmod(seconds, 24*3600)
             d += days
             s += seconds
-            microseconds = round(microseconds + usdouble)
+            microseconds = _round_half_up(microseconds + usdouble)
         assert isinstance(s, int)
         assert isinstance(microseconds, int)
         assert abs(s) <= 3 * 24 * 3600
@@ -1366,28 +1374,34 @@ class datetime(date):
         return self._tzinfo
 
     @classmethod
-    def fromtimestamp(cls, t, tz=None):
+    def _fromtimestamp(cls, t, utc, tz):
         """Construct a datetime from a POSIX timestamp (like time.time()).
 
         A timezone info object may be passed in as well.
         """
-        _check_tzinfo_arg(tz)
-
-        converter = _time.localtime if tz is None else _time.gmtime
-
-        t, frac = divmod(t, 1.0)
-        us = int(frac * 1e6)
-
-        # If timestamp is less than one microsecond smaller than a
-        # full second, us can be rounded up to 1000000.  In this case,
-        # roll over to seconds, otherwise, ValueError is raised
-        # by the constructor.
-        if us == 1000000:
+        frac, t = _math.modf(t)
+        us = _round_half_up(frac * 1e6)
+        if us >= 1000000:
             t += 1
-            us = 0
+            us -= 1000000
+        elif us < 0:
+            t -= 1
+            us += 1000000
+
+        converter = _time.gmtime if utc else _time.localtime
         y, m, d, hh, mm, ss, weekday, jday, dst = converter(t)
         ss = min(ss, 59)    # clamp out leap seconds if the platform has them
-        result = cls(y, m, d, hh, mm, ss, us, tz)
+        return cls(y, m, d, hh, mm, ss, us, tz)
+
+    @classmethod
+    def fromtimestamp(cls, t, tz=None):
+        """Construct a datetime from a POSIX timestamp (like time.time()).
+
+        A timezone info object may be passed in as well.
+        """
+        _check_tzinfo_arg(tz)
+
+        result = cls._fromtimestamp(t, tz is not None, tz)
         if tz is not None:
             result = tz.fromutc(result)
         return result
@@ -1395,19 +1409,7 @@ class datetime(date):
     @classmethod
     def utcfromtimestamp(cls, t):
         """Construct a naive UTC datetime from a POSIX timestamp."""
-        t, frac = divmod(t, 1.0)
-        us = int(frac * 1e6)
-
-        # If timestamp is less than one microsecond smaller than a
-        # full second, us can be rounded up to 1000000.  In this case,
-        # roll over to seconds, otherwise, ValueError is raised
-        # by the constructor.
-        if us == 1000000:
-            t += 1
-            us = 0
-        y, m, d, hh, mm, ss, weekday, jday, dst = _time.gmtime(t)
-        ss = min(ss, 59)    # clamp out leap seconds if the platform has them
-        return cls(y, m, d, hh, mm, ss, us)
+        return cls._fromtimestamp(t, True, None)
 
     @classmethod
     def now(cls, tz=None):
@@ -1918,6 +1920,8 @@ class timezone(tzinfo):
 
     @staticmethod
     def _name_from_offset(delta):
+        if not delta:
+            return 'UTC'
         if delta < timedelta(0):
             sign = '-'
             delta = -delta