diff --git a/Makefile.am b/Makefile.am
index d69a9ac33..b819688c9 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -517,6 +517,7 @@ libutil_a_SOURCES = \
 	src/util/TextFile.hxx \
 	src/util/TemplateString.hxx \
 	src/util/TimeParser.cxx src/util/TimeParser.hxx \
+	src/util/TimeConvert.cxx src/util/TimeConvert.hxx \
 	src/util/UriUtil.cxx src/util/UriUtil.hxx \
 	src/util/Manual.hxx \
 	src/util/StaticFifoBuffer.hxx \
diff --git a/src/TimePrint.cxx b/src/TimePrint.cxx
index 19115a6c6..646954224 100644
--- a/src/TimePrint.cxx
+++ b/src/TimePrint.cxx
@@ -20,20 +20,19 @@
 #include "config.h"
 #include "TimePrint.hxx"
 #include "client/Response.hxx"
+#include "util/TimeConvert.hxx"
 
 void
 time_print(Response &r, const char *name,
 	   std::chrono::system_clock::time_point t)
 {
-	time_t t2 = std::chrono::system_clock::to_time_t(t);
-#ifdef _WIN32
-	const struct tm *tm2 = gmtime(&t2);
-#else
 	struct tm tm;
-	const struct tm *tm2 = gmtime_r(&t2, &tm);
-#endif
-	if (tm2 == nullptr)
+
+	try {
+		tm = GmTime(t);
+	} catch (...) {
 		return;
+	}
 
 	char buffer[32];
 	strftime(buffer, sizeof(buffer),
@@ -42,6 +41,6 @@ time_print(Response &r, const char *name,
 #else
 		 "%FT%TZ",
 #endif
-		 tm2);
+		 &tm);
 	r.Format("%s: %s\n", name, buffer);
 }
diff --git a/src/tag/Format.cxx b/src/tag/Format.cxx
index d8aa2f3f3..ab49775f0 100644
--- a/src/tag/Format.cxx
+++ b/src/tag/Format.cxx
@@ -23,6 +23,7 @@
 #include "ParseName.hxx"
 #include "util/format.h"
 #include "util/TruncateString.hxx"
+#include "util/TimeConvert.hxx"
 
 #include <algorithm>
 
@@ -86,15 +87,13 @@ TagGetter(const void *object, const char *name) noexcept
 	auto &ctx = const_cast<FormatTagContext &>(_ctx);
 
 	if (strcmp(name, "iso8601") == 0) {
-		time_t t = time(nullptr);
-#ifdef _WIN32
-		const struct tm *tm2 = gmtime(&t);
-#else
 		struct tm tm;
-		const struct tm *tm2 = gmtime_r(&t, &tm);
-#endif
-		if (tm2 == nullptr)
+
+		try {
+			tm = GmTime(std::chrono::system_clock::now());
+		} catch (...) {
 			return "";
+		}
 
 		strftime(ctx.buffer, sizeof(ctx.buffer),
 #ifdef _WIN32
@@ -106,7 +105,7 @@ TagGetter(const void *object, const char *name) noexcept
 #else
 			 "%FT%TZ",
 #endif
-			 tm2);
+			 &tm);
 		return ctx.buffer;
 	}
 
diff --git a/src/util/TimeConvert.cxx b/src/util/TimeConvert.cxx
new file mode 100644
index 000000000..e0d506018
--- /dev/null
+++ b/src/util/TimeConvert.cxx
@@ -0,0 +1,83 @@
+/*
+ * Copyright 2007-2017 Content Management AG
+ * All rights reserved.
+ *
+ * author: Max Kellermann <mk@cm4all.com>
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the
+ * distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
+ * FOUNDATION OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
+ * OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include "TimeConvert.hxx"
+
+#include <stdexcept>
+
+#include <time.h>
+
+struct tm
+GmTime(std::chrono::system_clock::time_point tp)
+{
+	const time_t t = std::chrono::system_clock::to_time_t(tp);
+#ifdef _WIN32
+	const struct tm *tm = gmtime(&t);
+#else
+	struct tm buffer, *tm = gmtime_r(&t, &buffer);
+#endif
+	if (tm == nullptr)
+		throw std::runtime_error("gmtime_r() failed");
+
+	return *tm;
+}
+
+struct tm
+LocalTime(std::chrono::system_clock::time_point tp)
+{
+	const time_t t = std::chrono::system_clock::to_time_t(tp);
+#ifdef _WIN32
+	const struct tm *tm = localtime(&t);
+#else
+	struct tm buffer, *tm = localtime_r(&t, &buffer);
+#endif
+	if (tm == nullptr)
+		throw std::runtime_error("localtime_r() failed");
+
+	return *tm;
+}
+
+#ifdef __GLIBC__
+
+std::chrono::system_clock::time_point
+TimeGm(struct tm &tm)
+{
+	return std::chrono::system_clock::from_time_t(timegm(&tm));
+}
+
+#endif
+
+std::chrono::system_clock::time_point
+MakeTime(struct tm &tm)
+{
+	return std::chrono::system_clock::from_time_t(mktime(&tm));
+}
diff --git a/src/util/TimeConvert.hxx b/src/util/TimeConvert.hxx
new file mode 100644
index 000000000..5f75cd40f
--- /dev/null
+++ b/src/util/TimeConvert.hxx
@@ -0,0 +1,66 @@
+/*
+ * Copyright 2007-2017 Content Management AG
+ * All rights reserved.
+ *
+ * author: Max Kellermann <mk@cm4all.com>
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the
+ * distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
+ * FOUNDATION OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
+ * OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#ifndef TIME_CONVERT_HXX
+#define TIME_CONVERT_HXX
+
+#include <chrono>
+
+/**
+ * Convert a UTC-based time point to a UTC-based "struct tm".
+ */
+struct tm
+GmTime(std::chrono::system_clock::time_point tp);
+
+/**
+ * Convert a UTC-based time point to a local "struct tm".
+ */
+struct tm
+LocalTime(std::chrono::system_clock::time_point tp);
+
+#ifdef __GLIBC__
+
+/**
+ * Convert a UTC-based "struct tm" to a UTC-based time point.
+ */
+std::chrono::system_clock::time_point
+TimeGm(struct tm &tm);
+
+#endif
+
+/**
+ * Convert a local "struct tm" to a UTC-based time point.
+ */
+std::chrono::system_clock::time_point
+MakeTime(struct tm &tm);
+
+#endif