diff --git a/ext/date/date_core.c b/ext/date/date_core.c
index 0d69633..909d5b8 100644
--- a/ext/date/date_core.c
+++ b/ext/date/date_core.c
@@ -2636,8 +2636,17 @@ offset_to_sec(VALUE vof, int *rof)
 	    VALUE vs = date_zone_to_diff(vof);
 	    long n;
 
-	    if (!FIXNUM_P(vs))
-		return 0;
+	    if (!FIXNUM_P(vs)) {
+		VALUE vr;
+		if (!k_rational_p(vs))
+		    return 0;
+		vr = f_round(vs);
+		if (!f_eqeq_p(vr, vs))
+		    rb_warning("fraction of offset is ignored");
+		if (!FIXNUM_P(vr))
+		    return 0;
+		vs = vr;
+	    }
 	    n = FIX2LONG(vs);
 	    if (n < -DAY_IN_SECONDS || n > DAY_IN_SECONDS)
 		return 0;
diff --git a/test/date/test_date_new.rb b/test/date/test_date_new.rb
index b839c52..1d65a3b 100644
--- a/test/date/test_date_new.rb
+++ b/test/date/test_date_new.rb
@@ -213,6 +213,32 @@ def test_civil__offset
     assert_equal(0, d.offset)
   end
 
+  def test_civil__string_fractional_offset
+    # A fractional-hour zone with more than two digits yields a Rational from
+    # date_zone_to_diff; it must be rounded with a warning, the way the Float
+    # and Rational forms of the same quantity already are.
+    d = nil
+    assert_warning(/fraction of offset/) do
+      d = DateTime.civil(2001,2,3, 0,0,0, '+00.123')   # 442.8 seconds
+    end
+    assert_equal(443, (d.offset * 86400).to_i)
+    assert_equal(DateTime.civil(2001,2,3, 0,0,0, 442.8/86400).offset, d.offset)
+    assert_equal(DateTime.civil(2001,2,3, 0,0,0, Rational(2214, 5*86400)).offset, d.offset)
+
+    assert_warning(/fraction of offset/) do
+      d = DateTime.civil(2001,2,3, 0,0,0, '-00.123')
+    end
+    assert_equal(-443, (d.offset * 86400).to_i)
+
+    # Zones that are already an integral number of seconds are unaffected.
+    assert_equal(5400, (DateTime.civil(2001,2,3, 0,0,0, '+01.5').offset * 86400).to_i)
+    assert_equal(86399, (DateTime.civil(2001,2,3, 0,0,0, '+23:59:59').offset * 86400).to_i)
+
+    # Not a widening: a zone that date_zone_to_diff rejects stays rejected.
+    assert_equal(0, DateTime.civil(2001,2,3, 0,0,0, '+24:00').offset)
+    assert_equal(0, DateTime.civil(2001,2,3, 0,0,0, '+99:00').offset)
+  end
+
   def test_civil__reform
     d = Date.jd(Date::ENGLAND, Date::ENGLAND)
     dt = DateTime.jd(Date::ENGLAND, 0,0,0,0, Date::ENGLAND)