Feature 04 task 2: Household week and local-time utilities (#18)

Co-authored-by: dev_agent <dev_agent@cubecraftlabs.com>
Co-committed-by: dev_agent <dev_agent@cubecraftlabs.com>

internal/store/time.go

@@ -86,6 +86,17 @@ func ParseLocalDate(s string) (LocalDate, error) {
 	return LocalDate{t.Year(), t.Month(), t.Day()}, nil
 }
 
+// LocalDateOf returns the calendar date of t in t's own location.
+func LocalDateOf(t time.Time) LocalDate {
+	y, m, d := t.Date()
+	return LocalDate{y, m, d}
+}
+
+// Date returns the year, month and day of d.
+func (d LocalDate) Date() (year int, month time.Month, day int) {
+	return d.year, d.month, d.day
+}
+
 // String returns the YYYY-MM-DD form.
 func (d LocalDate) String() string {
 	return fmt.Sprintf("%04d-%02d-%02d", d.year, int(d.month), d.day)
@@ -96,11 +107,15 @@ func (d LocalDate) IsZero() bool {
 	return d == LocalDate{}
 }
 
-// Value implements driver.Valuer. The zero LocalDate is an error.
+// Value implements driver.Valuer. The zero LocalDate is an error, as is a
+// year outside 0000–9999, which the four-digit stored form cannot represent.
 func (d LocalDate) Value() (driver.Value, error) {
 	if d.IsZero() {
 		return nil, errors.New("store: zero LocalDate")
 	}
+	if d.year < 0 || d.year > 9999 {
+		return nil, fmt.Errorf("store: LocalDate year %d outside 0000-9999", d.year)
+	}
 	return d.String(), nil
 }
 

internal/store/time_test.go

@@ -102,6 +102,25 @@ func TestLocalDateRoundTrip(t *testing.T) {
 	}
 }
 
+func TestLocalDateOf(t *testing.T) {
+	auckland, err := time.LoadLocation("Pacific/Auckland")
+	if err != nil {
+		t.Fatal(err)
+	}
+	// 2026-10-03T20:00Z is already 2026-10-04 in Auckland (UTC+13).
+	instant := time.Date(2026, time.October, 3, 20, 0, 0, 0, time.UTC)
+	if got := LocalDateOf(instant).String(); got != "2026-10-03" {
+		t.Errorf("LocalDateOf(UTC) = %s, want 2026-10-03", got)
+	}
+	d := LocalDateOf(instant.In(auckland))
+	if got := d.String(); got != "2026-10-04" {
+		t.Errorf("LocalDateOf(Auckland) = %s, want 2026-10-04", got)
+	}
+	if y, m, day := d.Date(); y != 2026 || m != time.October || day != 4 {
+		t.Errorf("Date() = %d-%d-%d, want 2026-10-4", y, m, day)
+	}
+}
+
 func TestLocalDateRejectsInvalid(t *testing.T) {
 	for _, str := range []string{
 		"", "2026-02-30", "2026-02-29", "2026-13-01", "2026-00-10", "2026-10-00",
@@ -119,6 +138,12 @@ func TestLocalDateRejectsInvalid(t *testing.T) {
 	if _, err := (LocalDate{}).Value(); err == nil {
 		t.Error("zero LocalDate Value succeeded")
 	}
+	for _, y := range []int{-1, 10000} {
+		d := LocalDateOf(time.Date(y, time.January, 1, 0, 0, 0, 0, time.UTC))
+		if _, err := d.Value(); err == nil {
+			t.Errorf("LocalDate year %d Value succeeded", y)
+		}
+	}
 	var d LocalDate
 	if err := d.Scan(nil); err == nil {
 		t.Error("Scan(nil) succeeded")