diff --git a/lib/isc/tests/time_test.c b/lib/isc/tests/time_test.c
index 912127bda7..caddec2bec 100644
--- a/lib/isc/tests/time_test.c
+++ b/lib/isc/tests/time_test.c
@@ -15,6 +15,7 @@
  */
 
 #include <config.h>
+#include <stdlib.h>
 
 #include <atf-c.h>
 
@@ -30,6 +31,7 @@ ATF_TC_BODY(isc_time_parsehttptimestamp, tc) {
 	isc_time_t t, x;
 	char buf[100];
 
+	setenv("TZ", "PST8PDT", 1);
 	result = isc_time_now(&t);
 	ATF_REQUIRE_EQ(result, ISC_R_SUCCESS);
 
diff --git a/lib/isc/unix/time.c b/lib/isc/unix/time.c
index b39d47b00a..9039b06a88 100644
--- a/lib/isc/unix/time.c
+++ b/lib/isc/unix/time.c
@@ -23,6 +23,7 @@
 
 #include <errno.h>
 #include <limits.h>
+#include <stdlib.h>
 #include <syslog.h>
 #include <time.h>
 
@@ -407,6 +408,31 @@ isc_time_formathttptimestamp(const isc_time_t *t, char *buf, unsigned int len) {
 	INSIST(flen < len);
 }
 
+static time_t
+timetfromtm(struct tm *tm) {
+	time_t ret;
+	int i, yday = 0, leapday;
+	int mdays[] = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30 };
+
+	leapday = ((((tm->tm_year + 1900 ) % 4) == 0 &&
+		    ((tm->tm_year + 1900 ) % 100) != 0) ||
+		   ((tm->tm_year + 1900 ) % 400) == 0) ? 1 : 0;
+	mdays[1] += leapday;
+
+	yday = tm->tm_mday - 1;
+	for (i = 1; i <= tm->tm_mon; i++)
+		yday += mdays[i - 1];
+	ret = tm->tm_sec +
+	      (60 * tm->tm_min) +
+	      (3600 * tm->tm_hour) +
+	      (86400 * (yday +
+		       ((tm->tm_year - 70) * 365) +
+		       ((tm->tm_year - 69) / 4) -
+		       ((tm->tm_year - 1) / 100) +
+		       ((tm->tm_year + 299) / 400)));
+	return (ret);
+}
+
 isc_result_t
 isc_time_parsehttptimestamp(char *buf, isc_time_t *t) {
 	struct tm t_tm;
@@ -415,10 +441,10 @@ isc_time_parsehttptimestamp(char *buf, isc_time_t *t) {
 
 	REQUIRE(buf != NULL);
 	REQUIRE(t != NULL);
-	p = strptime(buf, "%a, %d %b %Y %H:%M:%S %Z", &t_tm);
+	p = strptime(buf, "%a, %d %b %Y %H:%M:%S", &t_tm);
 	if (p == NULL)
 		return (ISC_R_UNEXPECTED);
-	when = mktime(&t_tm);
+	when = timetfromtm(&t_tm);
 	if (when == -1)
 		return (ISC_R_UNEXPECTED);
 	isc_time_set(t, when, 0);
diff --git a/lib/isc/win32/time.c b/lib/isc/win32/time.c
index f2782001a2..d5049950bf 100644
--- a/lib/isc/win32/time.c
+++ b/lib/isc/win32/time.c
@@ -318,6 +318,31 @@ isc_time_formathttptimestamp(const isc_time_t *t, char *buf, unsigned int len) {
 	}
 }
 
+static time_t
+timetfromtm(struct tm *tm) {
+	time_t ret;
+	int i, yday = 0, leapday;
+	int mdays[] = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30 };
+
+	leapday = ((((tm->tm_year + 1900 ) % 4) == 0 &&
+		    ((tm->tm_year + 1900 ) % 100) != 0) ||
+		   ((tm->tm_year + 1900 ) % 400) == 0) ? 1 : 0;
+	mdays[1] += leapday;
+
+	yday = tm->tm_mday - 1;
+	for (i = 1; i <= tm->tm_mon; i++)
+		yday += mdays[i - 1];
+	ret = tm->tm_sec +
+	      (60 * tm->tm_min) +
+	      (3600 * tm->tm_hour) +
+	      (86400 * (yday +
+		       ((tm->tm_year - 70) * 365) +
+		       ((tm->tm_year - 69) / 4) -
+		       ((tm->tm_year - 1) / 100) +
+		       ((tm->tm_year + 299) / 400)));
+	return (ret);
+}
+
 #include "strptime.c"
 isc_result_t
 isc_time_parsehttptimestamp(char *buf, isc_time_t *t) {
@@ -330,7 +355,7 @@ isc_time_parsehttptimestamp(char *buf, isc_time_t *t) {
 	p = strptime(buf, "%a, %d %b %Y %H:%M:%S %Z", &t_tm);
 	if (p == NULL)
 		return (ISC_R_UNEXPECTED);
-	when = mktime(&t_tm);
+	when = timetfromtm(&t_tm);
 	if (when == -1)
 		return (ISC_R_UNEXPECTED);
 	isc_time_set(t, when, 0);