Bug fixed in Period.AddTo calculation: a more accurate solution is po…

…ssible

period/period.go

@@ -403,13 +403,19 @@ func (period Period) SecondsFloat() float32 {
 // result is precise. However, when years, months or days contains fractions, the result
 // is only an approximation (it assumes that all days are 24 hours and every year is 365.2425 days).
 func (period Period) AddTo(t time.Time) (time.Time, bool) {
-	d, precise := period.Duration()
-	if !precise {
-		return t.Add(d), false
+	wholeYears := (period.years % 10) == 0
+	wholeMonths := (period.months % 10) == 0
+	wholeDays := (period.days % 10) == 0
+
+	if wholeYears && wholeMonths && wholeDays {
+		// in this case, time.AddDate provides an exact solution
+		stE3 := totalSecondsE3(period)
+		t1 := t.AddDate(int(period.years/10), int(period.months/10), int(period.days/10))
+		return t1.Add(stE3 * time.Millisecond), true
 	}
 
-	stE3 := totalSecondsE3(period)
-	return t.AddDate(period.Years(), period.Months(), period.Days()).Add(stE3 * time.Millisecond), true
+	d, precise := period.Duration()
+	return t.Add(d), precise
 }
 
 // DurationApprox converts a period to the equivalent duration in nanoseconds.

period/period_test.go

@@ -225,6 +225,7 @@ func TestPeriodAddToTime(t *testing.T) {
 		result  time.Time
 		precise bool
 	}{
+		// precise cases
 		{"P0D", t0, true},
 		{"PT1S", t0.Add(sec), true},
 		{"PT0.1S", t0.Add(100 * ms), true},
@@ -236,16 +237,20 @@ func TestPeriodAddToTime(t *testing.T) {
 		{"PT1H", t0.Add(hr), true},
 		{"PT0.1H", t0.Add(6 * min), true},
 		{"PT3276H", t0.Add(3276 * hr), true},
-		{"P1D", t0.Add(24 * hr), false},
+		{"P1D", t0.AddDate(0, 0, 1), true},
+		{"P3276D", t0.AddDate(0, 0, 3276), true},
+		{"P1M", t0.AddDate(0, 1, 0), true},
+		{"P3276M", t0.AddDate(0, 3276, 0), true},
+		{"P1Y", t0.AddDate(1, 0, 0), true},
+		{"-P1Y", t0.AddDate(-1, 0, 0), true},
+		{"P3276Y", t0.AddDate(3276, 0, 0), true},   // near the upper limit of range
+		{"-P3276Y", t0.AddDate(-3276, 0, 0), true}, // near the lower limit of range
+		// approximate cases
 		{"P0.1D", t0.Add(144 * min), false},
-		{"P3276D", t0.Add(3276 * 24 * hr), false},
-		{"P1M", t0.Add(oneMonthApprox), false},
+		{"-P0.1D", t0.Add(-144 * min), false},
 		{"P0.1M", t0.Add(oneMonthApprox / 10), false},
-		{"P3276M", t0.Add(3276 * oneMonthApprox), false},
-		{"P1Y", t0.Add(oneYearApprox), false},
-		{"-P1Y", t0.Add(-oneYearApprox), false},
-		{"P3276Y", t0.Add(3276 * oneYearApprox), false},   // near the upper limit of range
-		{"-P3276Y", t0.Add(-3276 * oneYearApprox), false}, // near the lower limit of range
+		{"P0.1Y", t0.Add(oneYearApprox / 10), false},
+		{"-P0.1Y0.1M0.1D", t0.Add(-(oneYearApprox / 10) - (oneMonthApprox / 10) - (144 * min)), false},
 	}
 	for i, c := range cases {
 		p := MustParse(c.value)
@@ -254,7 +259,7 @@ func TestPeriodAddToTime(t *testing.T) {
 			t.Errorf("%d: AddTo(t) == %s %v, want %s for %s", i, t1, prec, c.result, c.value)
 		}
 		if prec != c.precise {
-			t.Errorf("%d: Duration() == %s %v, want %v for %s", i, t1, prec, c.precise, c.value)
+			t.Errorf("%d: AddTo(t) == %s %v, want %v for %s", i, t1, prec, c.precise, c.value)
 		}
 	}
 }