diff --git a/src/time/__tz.c b/src/time/__tz.c
index 54ed4cf6..e7ac0da7 100644
--- a/src/time/__tz.c
+++ b/src/time/__tz.c
@@ -436,3 +436,22 @@ const char *__tm_to_tzname(const struct tm *tm)
 	UNLOCK(lock);
 	return p;
 }
+
+int __tzname_to_isdst(const char **s)
+{
+	size_t len;
+	int isdst = -1;
+	LOCK(lock);
+	if (tzname[0] && !strncmp(*s, tzname[0], len = strlen(tzname[0]))) {
+		isdst = 0;
+		s += len;
+	} else if (tzname[1] && !strncmp(*s, tzname[1], len=strlen(tzname[1]))) {
+		isdst = 1;
+		s += len;
+	} else {
+		/* FIXME: is this supposed to be an error? */
+		while ((**s|32)-'a' <= 'z'-'a') ++*s;
+	}
+	UNLOCK(lock);
+	return isdst;
+}
diff --git a/src/time/strptime.c b/src/time/strptime.c
index b1147242..4a6df8b5 100644
--- a/src/time/strptime.c
+++ b/src/time/strptime.c
@@ -5,11 +5,12 @@
 #include <stddef.h>
 #include <string.h>
 #include <strings.h>
+#include "time_impl.h"
 
 char *strptime(const char *restrict s, const char *restrict f, struct tm *restrict tm)
 {
 	int i, w, neg, adj, min, range, *dest, dummy;
-	const char *ex;
+	const char *ex, *s1;
 	size_t len;
 	int want_century = 0, century = 0, relyear = 0;
 	while (*f) {
@@ -207,16 +208,10 @@ char *strptime(const char *restrict s, const char *restrict f, struct tm *restri
 			s += 5;
 			break;
 		case 'Z':
-			if (!strncmp(s, tzname[0], len = strlen(tzname[0]))) {
-				tm->tm_isdst = 0;
-				s += len;
-			} else if (!strncmp(s, tzname[1], len=strlen(tzname[1]))) {
-				tm->tm_isdst = 1;
-				s += len;
-			} else {
-				/* FIXME: is this supposed to be an error? */
-				while ((*s|32)-'a' <= 'z'-'a') s++;
-			}
+			s1 = s;
+			i = __tzname_to_isdst(&s1);
+			if (i>=0) tm->tm_isdst = i;
+			s = s1;
 			break;
 		case '%':
 			if (*s++ != '%') return 0;
diff --git a/src/time/time_impl.h b/src/time/time_impl.h
index f26d8005..5449a622 100644
--- a/src/time/time_impl.h
+++ b/src/time/time_impl.h
@@ -5,6 +5,7 @@ hidden int __month_to_secs(int, int);
 hidden long long __year_to_secs(long long, int *);
 hidden long long __tm_to_secs(const struct tm *);
 hidden const char *__tm_to_tzname(const struct tm *);
+hidden int __tzname_to_isdst(const char **);
 hidden int __secs_to_tm(long long, struct tm *);
 hidden void __secs_to_zone(long long, int, int *, long *, long *, const char **);
 hidden const char *__strftime_fmt_1(char (*)[100], size_t *, int, const struct tm *, locale_t, int);