lib: Fix some issues in timeutils

In particular, dates prior to Mar 1, 2000 are screwed up.

The easiest way to see this is to do:

>>> import time
>>> time.localtime(0)
(2000, 1, 1, 0, 0, 0, 5, 1)
>>> time.localtime(1)
(2000, 1, 2, 233, 197, 197, 6, 2)

With this patch, we instead get:
>>> import time
>>> time.localtime(1)
(2000, 1, 1, 0, 0, 1, 5, 1)

Doh - In C % is NOT a modulo operator, it's a remainder operator.

lib/timeutils/timeutils.c

@@ -73,6 +73,10 @@ void timeutils_seconds_since_2000_to_struct_time(mp_uint_t t, timeutils_struct_t
 
     mp_int_t days = seconds / 86400;
     seconds %= 86400;
+    if (seconds < 0) {
+        seconds += 86400;
+        days -= 1;
+    }
     tm->tm_hour = seconds / 3600;
     tm->tm_min = seconds / 60 % 60;
     tm->tm_sec = seconds % 60;