diff --git a/libc/bionic/time64.c b/libc/bionic/time64.c
index 95dfab5..da38bf3 100644
--- a/libc/bionic/time64.c
+++ b/libc/bionic/time64.c
@@ -748,10 +748,24 @@
 char *asctime64_r( const struct TM* date, char *result ) {
     /* I figure everything else can be displayed, even hour 25, but if
        these are out of range we walk off the name arrays */
-    if( !valid_tm_wday(date) || !valid_tm_mon(date) )
+    if (!valid_tm_wday(date) || !valid_tm_mon(date)) {
         return NULL;
+    }
 
-    sprintf(result, "%.3s %.3s%3d %.2d:%.2d:%.2d %d\n",
+    /* Docs state this function does not support years beyond 9999. */
+    if (1900 + date->tm_year > 9999) {
+        return NULL;
+    }
+
+    /*
+     * The IBM docs for this function state that the result buffer can be
+     * assumed to be at least 26 bytes wide. The docs also state that this is
+     * only valid for years <= 9999, so we know this format string will not
+     * print more than that many characters.
+     *
+     * http://www-01.ibm.com/support/knowledgecenter/SSLTBW_2.1.0/com.ibm.zos.v2r1.bpxbd00/asctimer.htm
+     */
+    snprintf(result, 26, "%.3s %.3s%3d %.2d:%.2d:%.2d %d\n",
         wday_name[date->tm_wday],
         mon_name[date->tm_mon],
         date->tm_mday, date->tm_hour,