diff --git a/duration.go b/duration.go
index cf9391f7..7851d103 100644
--- a/duration.go
+++ b/duration.go
@@ -47,8 +47,8 @@ func (d Duration) MarshalText() ([]byte, error) {
 
 const (
 	day   = 24 * time.Hour
-	month = 30 * day   // Assumed to be 30 days.
-	year  = 12 * month // Assumed to be non-leap year.
+	month = 30 * day  // Assumed to be 30 days.
+	year  = 365 * day // Assumed to be non-leap year.
 )
 
 var (
diff --git a/duration_test.go b/duration_test.go
index 29ea82f4..7302ed2c 100644
--- a/duration_test.go
+++ b/duration_test.go
@@ -46,12 +46,12 @@ var durationUnmarshalTests = []struct {
 	{[]byte("PT1M"), time.Minute, nil},
 	{[]byte("PT1H"), time.Hour, nil},
 	{[]byte("-PT1H"), -time.Hour, nil},
-	{[]byte("P1D"), day, nil},
-	{[]byte("P1M"), month, nil},
-	{[]byte("P1Y"), year, nil},
-	{[]byte("P2Y3M4DT5H6M7.000000008S"), 2*year + 3*month + 4*day + 5*time.Hour + 6*time.Minute + 7*time.Second + 8*time.Nanosecond, nil},
+	{[]byte("P1D"), 24 * time.Hour, nil},
+	{[]byte("P1M"), 720 * time.Hour, nil},
+	{[]byte("P1Y"), 8760 * time.Hour, nil},
+	{[]byte("P2Y3M4DT5H6M7.000000008S"), 19781*time.Hour + 6*time.Minute + 7*time.Second + 8*time.Nanosecond, nil},
 	{[]byte("P0Y0M0DT0H0M0S"), 0, nil},
-	{[]byte("P0001Y"), year, nil},
+	{[]byte("PT0001.0000S"), time.Second, nil},
 	{[]byte(""), 0, errors.New("invalid duration ()")},
 	{[]byte("12345"), 0, errors.New("invalid duration (12345)")},
 	{[]byte("P1D1M1Y"), 0, errors.New("invalid duration (P1D1M1Y)")},