time/Convert: fallback TimeGm() implementation

Move code from Parser.cxx.

src/time/Parser.cxx

@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2014-2017 Max Kellermann <max.kellermann@gmail.com>
+ * Copyright 2014-2019 Max Kellermann <max.kellermann@gmail.com>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -29,31 +29,12 @@
 
 #include "Parser.hxx"
 #include "Convert.hxx"
-#include "util/Compiler.h"
 
 #include <stdexcept>
 
 #include <assert.h>
 #include <time.h>
 
-#if !defined(__GLIBC__) && !defined(_WIN32)
-
-/**
- * Determine the time zone offset in a portable way.
- */
-gcc_const
-static time_t
-GetTimeZoneOffset() noexcept
-{
-	time_t t = 1234567890;
-	struct tm tm;
-	tm.tm_isdst = 0;
-	gmtime_r(&t, &tm);
-	return t - mktime(&tm);
-}
-
-#endif
-
 std::chrono::system_clock::time_point
 ParseTimePoint(const char *s, const char *format)
 {
@@ -71,13 +52,6 @@ ParseTimePoint(const char *s, const char *format)
 	if (end == nullptr || *end != 0)
 		throw std::runtime_error("Failed to parse time stamp");
 
-#ifdef __GLIBC__
-	/* timegm() is a GNU extension */
 	return TimeGm(tm);
-#else
-	tm.tm_isdst = 0;
-	const auto t = mktime(&tm) + GetTimeZoneOffset();
-	return std::chrono::system_clock::from_time_t(t);
-#endif /* !__GLIBC__ */
 #endif /* !_WIN32 */
 }