diff --git a/chrono/datetime.cpp b/chrono/datetime.cpp
index 3929385..0ed9984 100644
--- a/chrono/datetime.cpp
+++ b/chrono/datetime.cpp
@@ -146,7 +146,8 @@ std::pair<DateTime, TimeSpan> DateTime::fromIsoString(const char *str)
         } else if (c == '-') {
             if (valueIndex < dayIndex) {
                 ++valueIndex;
-            } else if (++valueIndex == deltaHourIndex) {
+            } else if (++valueIndex >= secondsIndex) {
+                valueIndex = deltaHourIndex;
                 deltaNegative = true;
             } else {
                 throw ConversionException("unexpected \"-\" after day");
diff --git a/tests/chronotests.cpp b/tests/chronotests.cpp
index 7c92ea2..223d00d 100644
--- a/tests/chronotests.cpp
+++ b/tests/chronotests.cpp
@@ -177,8 +177,11 @@ void ChronoTests::testDateTime()
     CPPUNIT_ASSERT_EQUAL_MESSAGE("Zulu time", TimeSpan(), DateTime::fromIsoString("2017-08-23T19:40:15.985077682Z").second);
     CPPUNIT_ASSERT_EQUAL_MESSAGE("no minutes", TimeSpan::fromHours(3), DateTime::fromIsoString("2017-08-23T19:40:15.985077682+03").second);
     const auto test6 = DateTime::fromIsoString("1970-01-01T01:02:03+01:00");
-    CPPUNIT_ASSERT_EQUAL_MESSAGE("no seconds fraction", DateTime::fromDateAndTime(1970, 1, 1, 1, 2, 3), test6.first);
-    CPPUNIT_ASSERT_EQUAL_MESSAGE("no seconds fraction", TimeSpan::fromHours(1.0), test6.second);
+    CPPUNIT_ASSERT_EQUAL_MESSAGE("no seconds fraction (positive timezone offset, 1)", DateTime::fromDateAndTime(1970, 1, 1, 1, 2, 3), test6.first);
+    CPPUNIT_ASSERT_EQUAL_MESSAGE("no seconds fraction (positive timezone offset, 2)", TimeSpan::fromHours(1.0), test6.second);
+    const auto test7 = DateTime::fromIsoString("2021-05-20T23:02:45-04:00");
+    CPPUNIT_ASSERT_EQUAL_MESSAGE("no seconds fraction (negative timezone offset, 1)", DateTime::fromDateAndTime(2021, 5, 20, 23, 2, 45), test7.first);
+    CPPUNIT_ASSERT_EQUAL_MESSAGE("no seconds fraction (negative timezone offset, 2)", TimeSpan::fromHours(-4.0), test7.second);
     // test invalid characters
     CPPUNIT_ASSERT_THROW_MESSAGE("digits after Z", DateTime::fromIsoString("2017-O8-23T19:40:15.985077682Z02:00"), ConversionException);
     CPPUNIT_ASSERT_THROW_MESSAGE("invalid letter", DateTime::fromIsoString("2017-O8-23T19:40:15.985077682:+02:00"), ConversionException);