util: add support for other NTP eras

NTP timestamps use only 32 bits to count seconds and the current NTP era
ends in 2036. Add support for converting NTP timestamps from other NTP
eras on systems with 64-bit time_t.

The earliest assumed NTP time is set by the configure script (by default
to 50 years before the date of the build) and earlier NTP timestamps
underflow to the following NTP era.

ntp_core.c

@@ -258,10 +258,35 @@ do_size_checks(void)
 
 /* ================================================== */
 
+static void
+do_time_checks(void)
+{
+#ifdef HAVE_LONG_TIME_T
+  /* Check that time before NTP_ERA_SPLIT underflows correctly */
+
+  struct timeval tv1 = {NTP_ERA_SPLIT, 1}, tv2 = {NTP_ERA_SPLIT - 1, 1};
+  NTP_int64 ntv1, ntv2;
+  int r;
+
+  UTI_TimevalToInt64(&tv1, &ntv1, 0);
+  UTI_TimevalToInt64(&tv2, &ntv2, 0);
+  UTI_Int64ToTimeval(&ntv1, &tv1);
+  UTI_Int64ToTimeval(&ntv2, &tv2);
+
+  r = tv1.tv_sec == NTP_ERA_SPLIT &&
+      tv1.tv_sec + (1ULL << 32) - 1 == tv2.tv_sec;
+
+  assert(r);
+#endif
+}
+
+/* ================================================== */
+
 void
 NCR_Initialise(void)
 {
   do_size_checks();
+  do_time_checks();
 
   logfileid = CNF_GetLogMeasurements() ? LOG_FileOpen("measurements",
       "   Date (UTC) Time     IP Address   L St 1234 abc 5678 LP RP Score Offset     Peer del. Peer disp. Root del.  Root disp.")