rotate: don't throw away microseconds calculating threshold

Throwing away the microseconds from the rotation threshold doesn't affect real
world usage but it does make tests unreliable because it means the outcome is
sensitive to exactly which part of a second the test is running in and if near
the rollover point then a different result is obtained.

lib/sqlite.c

@@ -526,11 +526,11 @@ sqlite_rotate(const char *db_path, const int days, char **wtmpdb_name,
   sqlite3 *db_src;
   sqlite3 *db_dest;
   uint64_t counter = 0;
-  struct timespec ts_now;
-  clock_gettime (CLOCK_REALTIME, &ts_now);
-  time_t offset = ts_now.tv_sec - days * 86400;
-  struct tm *tm = localtime (&offset);
-  uint64_t login_t = offset * USEC_PER_SEC;
+  struct timespec threshold;
+  clock_gettime (CLOCK_REALTIME, &threshold);
+  threshold.tv_sec -= days * 86400;
+  struct tm *tm = localtime (&threshold.tv_sec);
+  uint64_t login_t = wtmpdb_timespec2usec (threshold);
   char date[10];
   strftime (date, 10, "%Y%m%d", tm);
   char *dest_path = NULL;