update: use KnownApks dates to check system clock on offline machines

KnownApks provides a reliable source of a relatively recent date.

fdroidserver/common.py

@@ -40,7 +40,7 @@ import json
 import xml.etree.ElementTree as XMLElementTree
 
 from binascii import hexlify
-from datetime import datetime
+from datetime import datetime, timedelta
 from distutils.version import LooseVersion
 from queue import Queue
 from zipfile import ZipFile
@@ -1716,6 +1716,23 @@ def natural_key(s):
     return [int(sp) if sp.isdigit() else sp for sp in re.split(r'(\d+)', s)]
 
 
+def check_system_clock(dt_obj, path):
+    """Check if system clock is updated based on provided date
+
+    If an APK has files newer than the system time, suggest updating
+    the system clock.  This is useful for offline systems, used for
+    signing, which do not have another source of clock sync info. It
+    has to be more than 24 hours newer because ZIP/APK files do not
+    store timezone info
+
+    """
+    checkdt = dt_obj - timedelta(1)
+    if datetime.today() < checkdt:
+        logging.warning(_('System clock is older than date in {path}!').format(path=path)
+                        + '\n' + _('Set clock to that time using:') + '\n'
+                        + 'sudo date -s "' + str(dt_obj) + '"')
+
+
 class KnownApks:
     """permanent store of existing APKs with the date they were added
 
@@ -1744,6 +1761,7 @@ class KnownApks:
                         date = datetime.strptime(t[-1], '%Y-%m-%d')
                         filename = line[0:line.rfind(appid) - 1]
                         self.apks[filename] = (appid, date)
+                        check_system_clock(date, self.path)
         self.changed = False
 
     def writeifchanged(self):