Time conversion functions with tests.

Fixes issue with incorrect characters in time strings on UI.
Fix platform dependency


Fix return value with incorrect strings.


Just use strptime and strftime on all platforms.

Emulate strptime on msvc... because they don't have it and their get_time is buggy.

tests/timeutils/timeutils_tests_main.cpp

@@ -0,0 +1,41 @@
+#include <gtest/gtest.h>
+#include "libslic3r/Time.hpp"
+
+#include <sstream>
+#include <iomanip>
+#include <locale>
+
+namespace {
+
+void test_time_fmt(Slic3r::Utils::TimeFormat fmt) {
+    using namespace Slic3r::Utils;
+    time_t t = get_current_time_utc();
+    
+    std::string tstr = time2str(t, TimeZone::local, fmt);
+    time_t parsedtime = str2time(tstr, TimeZone::local, fmt);
+    ASSERT_EQ(t, parsedtime);
+    
+    tstr = time2str(t, TimeZone::utc, fmt);
+    parsedtime = str2time(tstr, TimeZone::utc, fmt);
+    ASSERT_EQ(t, parsedtime);
+    
+    parsedtime = str2time("not valid string", TimeZone::local, fmt);
+    ASSERT_EQ(parsedtime, time_t(-1));
+    
+    parsedtime = str2time("not valid string", TimeZone::utc, fmt);
+    ASSERT_EQ(parsedtime, time_t(-1));
+}
+}
+
+TEST(Timeutils, ISO8601Z) {
+    test_time_fmt(Slic3r::Utils::TimeFormat::iso8601Z);
+}
+
+TEST(Timeutils, Slic3r_UTC_Time_Format) {
+    test_time_fmt(Slic3r::Utils::TimeFormat::gcode);
+}
+
+int main(int argc, char **argv) {
+    ::testing::InitGoogleTest(&argc, argv);
+    return RUN_ALL_TESTS();
+}