makedate: wrong timezone offset if DST rules changed this year (issue2511)

Python's time module sets timezone and altzone based on UTC offsets of
two dates: first and middle day of the current year. This approach
doesn't work on a year when DST rules change.

For example Russia abandoned winter time this year, so the correct UTC
offset should be +4 now, but time.timezone returns 3 hours difference
because that's what it was on 01.01.2011.

Related python issue: http://bugs.python.org/issue1647654

mercurial/util.py

@@ -16,7 +16,7 b' hide platform-specific details from the '
 from i18n import _
 import error, osutil, encoding
 import errno, re, shutil, sys, tempfile, traceback
-import os, time, calendar, textwrap, signal
+import os, time, datetime, calendar, textwrap, signal
 import imp, socket, urllib
 
 if os.name == 'nt':
@@ -900,16 +900,14 b' def filechunkiter(f, size=65536, limit=N'
         yield s
 
 def makedate():
-    lt = time.localtime()
-    if lt[8] == 1 and time.daylight:
-        tz = time.altzone
-    else:
-        tz = time.timezone
-    t = time.mktime(lt)
-    if t < 0:
+    ct = time.time()
+    if ct < 0:
         hint = _("check your clock")
-        raise Abort(_("negative timestamp: %d") % t, hint=hint)
-    return t, tz
+        raise Abort(_("negative timestamp: %d") % ct, hint=hint)
+    delta = (datetime.datetime.utcfromtimestamp(ct) -
+             datetime.datetime.fromtimestamp(ct))
+    tz = delta.days * 86400 + delta.seconds
+    return ct, tz
 
 def datestr(date=None, format='%a %b %d %H:%M:%S %Y %1%2'):
     """represent a (unixtime, offset) tuple as a localized time.