ntptime: Fix Year 2036 bug.

Fix NTP client - it would report the wrong time after 7 Feb 2036.

Signed-off-by: Jon Foster <jon@jon-foster.co.uk>

micropython/net/ntptime/manifest.py

@@ -1,3 +1,3 @@
-metadata(description="NTP client.", version="0.1.0")
+metadata(description="NTP client.", version="0.1.1")
 
 module("ntptime.py", opt=3)

micropython/net/ntptime/ntptime.py

@@ -22,12 +22,37 @@ def time():
     s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
     try:
         s.settimeout(timeout)
-        res = s.sendto(NTP_QUERY, addr)
+        s.sendto(NTP_QUERY, addr)
         msg = s.recv(48)
     finally:
         s.close()
     val = struct.unpack("!I", msg[40:44])[0]
 
+    # 2024-01-01 00:00:00 converted to an NTP timestamp
+    MIN_NTP_TIMESTAMP = 3913056000
+
+    # Y2036 fix
+    #
+    # The NTP timestamp has a 32-bit count of seconds, which will wrap back
+    # to zero on 7 Feb 2036 at 06:28:16.
+    #
+    # We know that this software was written during 2024 (or later).
+    # So we know that timestamps less than MIN_NTP_TIMESTAMP are impossible.
+    # So if the timestamp is less than MIN_NTP_TIMESTAMP, that probably means
+    # that the NTP time wrapped at 2^32 seconds.  (Or someone set the wrong
+    # time on their NTP server, but we can't really do anything about that).
+    #
+    # So in that case, we need to add in those extra 2^32 seconds, to get the
+    # correct timestamp.
+    #
+    # This means that this code will work until the year 2160.  More precisely,
+    # this code will not work after 7th Feb 2160 at 06:28:15.
+    #
+    if val < MIN_NTP_TIMESTAMP:
+        val += 0x100000000
+
+    # Convert timestamp from NTP format to our internal format
+
     EPOCH_YEAR = utime.gmtime(0)[0]
     if EPOCH_YEAR == 2000:
         # (date(2000, 1, 1) - date(1900, 1, 1)).days * 24*60*60