Fix month parsing overflowing into the next month

rules/context.go

@@ -29,7 +29,16 @@ func (c *Context) Time(t time.Time) (time.Time, error) {
 	}
 
 	if c.Month != nil {
-		t = time.Date(t.Year(), time.Month(*c.Month), t.Day(),
+		month := time.Month(*c.Month)
+
+		// time.Date would normalize a day the target month lacks into the next
+		// one; clamp instead.
+		day := t.Day()
+		if last := daysIn(month, t.Year()); day > last {
+			day = last
+		}
+
+		t = time.Date(t.Year(), month, day,
 			t.Hour(), t.Minute(), t.Second(), t.Nanosecond(), t.Location())
 	}
 
@@ -66,3 +75,8 @@ func (c *Context) Time(t time.Time) (time.Time, error) {
 
 	return t, nil
 }
+
+// daysIn returns the number of days in the given month of the given year.
+func daysIn(m time.Month, year int) int {
+	return time.Date(year, m+1, 0, 0, 0, 0, 0, time.UTC).Day()
+}

rules/en/exact_month_date_test.go

@@ -7,6 +7,7 @@ import (
 	"github.com/olebedev/when"
 	"github.com/olebedev/when/rules"
 	"github.com/olebedev/when/rules/en"
+	"github.com/stretchr/testify/require"
 )
 
 func TestExactMonthDate(t *testing.T) {
@@ -37,3 +38,32 @@ func TestExactMonthDate(t *testing.T) {
 
 	ApplyFixtures(t, "en.ExactMonthDate", w, fixtok)
 }
+
+// Parsed against the 31st, a shorter target month used to overflow into the
+// next one: "september 5th" resolved to the 5th of October.
+func TestExactMonthDateFromLongMonth(t *testing.T) {
+	w := when.New(nil)
+	w.Add(en.ExactMonthDate(rules.Override))
+
+	ref := time.Date(2016, time.October, 31, 0, 0, 0, 0, time.UTC)
+
+	fixt := []struct {
+		Text     string
+		Expected time.Time
+	}{
+		{"september 5th", time.Date(2016, time.September, 5, 0, 0, 0, 0, time.UTC)},
+		{"february 11", time.Date(2016, time.February, 11, 0, 0, 0, 0, time.UTC)},
+		{"1st of june", time.Date(2016, time.June, 1, 0, 0, 0, 0, time.UTC)},
+		// No day in the text: the reference day clamps to the month's last.
+		{"september", time.Date(2016, time.September, 30, 0, 0, 0, 0, time.UTC)},
+		// Months that do have a 31st keep the reference day untouched.
+		{"december", time.Date(2016, time.December, 31, 0, 0, 0, 0, time.UTC)},
+	}
+
+	for i, f := range fixt {
+		res, err := w.Parse(f.Text, ref)
+		require.Nil(t, err, "[en.ExactMonthDate] err #%d", i)
+		require.NotNil(t, res, "[en.ExactMonthDate] res #%d", i)
+		require.Equal(t, f.Expected, res.Time, "[en.ExactMonthDate] time #%d", i)
+	}
+}