time: midnight is 12AM.

This is the other half of the problem fixed at noon by the previous change.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/4515150

src/pkg/time/format.go

@@ -272,18 +272,19 @@ func (t *Time) Format(layout string) string {
 		case stdHour:
 			p = zeroPad(t.Hour)
 		case stdHour12:
-			// Noon is 12PM.
-			if t.Hour == 12 {
-				p = "12"
-			} else {
-				p = strconv.Itoa(t.Hour % 12)
+			// Noon is 12PM, midnight is 12AM.
+			hr := t.Hour % 12
+			if hr == 0 {
+				hr = 12
 			}
+			p = strconv.Itoa(hr)
 		case stdZeroHour12:
-			if t.Hour == 12 {
-				p = "12"
-			} else {
-				p = zeroPad(t.Hour % 12)
+			// Noon is 12PM, midnight is 12AM.
+			hr := t.Hour % 12
+			if hr == 0 {
+				hr = 12
 			}
+			p = zeroPad(hr)
 		case stdMinute:
 			p = strconv.Itoa(t.Minute)
 		case stdZeroMinute:
@@ -438,6 +439,7 @@ func skip(value, prefix string) (string, os.Error) {
 func Parse(alayout, avalue string) (*Time, os.Error) {
 	var t Time
 	rangeErrString := "" // set if a value is out of range
+	amSet := false       // do we need to subtract 12 from the hour for midnight?
 	pmSet := false       // do we need to add 12 to the hour?
 	layout, value := alayout, avalue
 	// Each iteration processes one std value.
@@ -567,9 +569,12 @@ func Parse(alayout, avalue string) (*Time, os.Error) {
 				break
 			}
 			p, value = value[0:2], value[2:]
-			if p == "PM" {
+			switch p {
+			case "PM":
 				pmSet = true
-			} else if p != "AM" {
+			case "AM":
+				amSet = true
+			default:
 				err = errBad
 			}
 		case stdpm:
@@ -578,9 +583,12 @@ func Parse(alayout, avalue string) (*Time, os.Error) {
 				break
 			}
 			p, value = value[0:2], value[2:]
-			if p == "pm" {
+			switch p {
+			case "pm":
 				pmSet = true
-			} else if p != "am" {
+			case "am":
+				amSet = true
+			default:
 				err = errBad
 			}
 		case stdTZ:
@@ -622,6 +630,8 @@ func Parse(alayout, avalue string) (*Time, os.Error) {
 	}
 	if pmSet && t.Hour < 12 {
 		t.Hour += 12
+	} else if amSet && t.Hour == 12 {
+		t.Hour = 0
 	}
 	return &t, nil
 }

src/pkg/time/time_test.go

@@ -315,6 +315,19 @@ func TestNoonIs12PM(t *testing.T) {
 	}
 }
 
+func TestMidnightIs12AM(t *testing.T) {
+	midnight := Time{Hour: 0}
+	expect := "12:00AM"
+	got := midnight.Format("3:04PM")
+	if got != expect {
+		t.Errorf("got %q; expect %q", got, expect)
+	}
+	got = midnight.Format("03:04PM")
+	if got != expect {
+		t.Errorf("got %q; expect %q", got, expect)
+	}
+}
+
 func Test12PMIsNoon(t *testing.T) {
 	noon, err := Parse("3:04PM", "12:00PM")
 	if err != nil {
@@ -332,6 +345,23 @@ func Test12PMIsNoon(t *testing.T) {
 	}
 }
 
+func Test12AMIsMidnight(t *testing.T) {
+	midnight, err := Parse("3:04PM", "12:00AM")
+	if err != nil {
+		t.Fatal("error parsing date:", err)
+	}
+	if midnight.Hour != 0 {
+		t.Errorf("got %d; expect 0", midnight.Hour)
+	}
+	midnight, err = Parse("03:04PM", "12:00AM")
+	if err != nil {
+		t.Fatal("error parsing date:", err)
+	}
+	if midnight.Hour != 0 {
+		t.Errorf("got %d; expect 0", midnight.Hour)
+	}
+}
+
 // Check that a time without a Zone still produces a (numeric) time zone
 // when formatted with MST as a requested zone.
 func TestMissingZone(t *testing.T) {