Implement gnc_timegm directly without mktime.

Calculate the seconds since the start of the UNIX epoch directly without
the help of mktime. This also works on Windows. Systems with timegm are
unaffected.


git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@15472 57a11ea4-9604-0410-9ed3-97b8803252fd

src/backend/file/sixtp-utils.c

@@ -381,35 +381,36 @@ simple_chars_only_parser_new(sixtp_end_handler end_handler)
 
 #ifdef HAVE_TIMEGM
 #  define gnc_timegm timegm
-#else
+#else /* !HAVE_TIMEGM */
+
+/* This code originates from GLib 2.12, gtimer.c and works until the year 2100
+ * or the system-dependent maximal date that can be represented by a time_t,
+ * whatever comes first.  The old implementation called mktime after setting
+ * the environment variable TZ to UTC.  It did not work on Windows, at least.
+ */
+static const gint days_before[] = {
+  0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334
+};
+
 static time_t
 gnc_timegm (struct tm *tm)
 {
-  time_t result;
-  char *old_tz;
-
-  old_tz = g_strdup(g_getenv ("TZ"));
-  /* FIXME: there's no way to report this error to the caller. */
-  if (!g_setenv("TZ", "UTC", TRUE))
-    PERR ("couldn't switch the TZ.");
-  result = mktime (tm);
-  if(old_tz)
-  {
-    /* FIXME: there's no way to report this error to the caller. */
-    if (!g_setenv("TZ", old_tz, TRUE))
-      PERR ("couldn't switch the TZ back.");
-    g_free(old_tz);
-  }
-  else
-  {
-    /* FIXME: there's no way to report this error to the caller. */
-    g_unsetenv("TZ");
-    if(errno != 0)
-      PERR ("couldn't restore the TZ to undefined.");
-  }
-  return result;
+  time_t retval;
+  if (tm->tm_mon < 0 || tm->tm_mon > 11)
+    return (time_t) -1;
+
+  retval = (tm->tm_year - 70) * 365;
+  retval += (tm->tm_year - 68) / 4;
+  retval += days_before[tm->tm_mon] + tm->tm_mday - 1;
+
+  if (tm->tm_year % 4 == 0 && tm->tm_mon < 2)
+    retval -= 1;
+  
+  retval = ((((retval * 24) + tm->tm_hour) * 60) + tm->tm_min) * 60 + tm->tm_sec;
+
+  return retval;
 }
-#endif
+#endif /* HAVE_TIMEGM */
 
 
 /****************************************************************************/