lavf: Implement ff_brktimegm using gmtime_r

While a standalone implementation is nice, we already depend on
gmtime and gmtime_r in a number of places.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/cutils.c

@@ -18,6 +18,8 @@
  * License along with Libav; if not, write to the Free Software
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
+
+#include "libavutil/time_internal.h"
 #include "avformat.h"
 #include "internal.h"
 
@@ -49,30 +51,10 @@ void ff_dynarray_add(intptr_t **tab_ptr, int *nb_ptr, intptr_t elem)
    couple of places, though. */
 struct tm *ff_brktimegm(time_t secs, struct tm *tm)
 {
-    int days, y, ny, m;
-    int md[] = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
-
-    days = secs / 86400;
-    secs %= 86400;
-    tm->tm_hour = secs / 3600;
-    tm->tm_min = (secs % 3600) / 60;
-    tm->tm_sec =  secs % 60;
-
-    /* oh well, may be someone some day will invent a formula for this stuff */
-    y = 1970; /* start "guessing" */
-    while (days > 365) {
-        ny = (y + days/366);
-        days -= (ny - y) * 365 + LEAPS_COUNT(ny - 1) - LEAPS_COUNT(y - 1);
-        y = ny;
-    }
-    if (days==365 && !ISLEAP(y)) { days=0; y++; }
-    md[1] = ISLEAP(y)?29:28;
-    for (m=0; days >= md[m]; m++)
-         days -= md[m];
+    tm = gmtime_r(&secs, tm);
 
-    tm->tm_year = y;  /* unlike gmtime_r we store complete year here */
-    tm->tm_mon = m+1; /* unlike gmtime_r tm_mon is from 1 to 12 */
-    tm->tm_mday = days+1;
+    tm->tm_year += 1900; /* unlike gmtime_r we store complete year here */
+    tm->tm_mon  += 1;    /* unlike gmtime_r tm_mon is from 1 to 12 */
 
     return tm;
 }