fix(deadline): reject day/month dates that read differently by locale

--deadline 03/04/2026 was silently parsed as March 4. Numeric dates such
as 03/04/2026 or 15.01.2028 are now refused with "the order of day and
month is ambiguous; use YYYY-MM-DD", and the error for other invalid
values no longer suggests clearing on issue create.

Signed-off-by: bircni <bircni@icloud.com>

internal/app/deadline.go

@@ -5,6 +5,8 @@ package app
 
 import (
 	"errors"
+	"fmt"
+	"regexp"
 	"strconv"
 	"time"
 
@@ -12,18 +14,25 @@ import (
 	"github.com/araddon/dateparse"
 )
 
+// numericDate matches dates such as 03/04/2026, which read as March 4 in the US and April 3
+// elsewhere.
+var numericDate = regexp.MustCompile(`^\d{1,2}[/.]\d{1,2}[/.]\d{2,4}$`)
+
 // parseDeadline parses a --deadline value; an empty value requests clearing.
 // Supplied timezone offsets are kept and Unix timestamps are UTC.
 func parseDeadline(value string) (*time.Time, *bool, error) {
 	if value == "" {
 		return nil, new(true), nil
 	}
+	if numericDate.MatchString(value) {
+		return nil, nil, clierr.Wrap(clierr.Usage, fmt.Errorf("invalid --deadline %q: the order of day and month is ambiguous; use YYYY-MM-DD", value))
+	}
 	date, err := dateparse.ParseAny(value)
 	if _, numericError := strconv.ParseInt(value, 10, 64); numericError == nil {
 		date = date.UTC()
 	}
 	if err != nil || date.IsZero() || date.Unix() == 0 || date.Year() < 1 || date.Year() > 9999 {
-		return nil, nil, clierr.Wrap(clierr.Usage, errors.New("invalid --deadline; use YYYY-MM-DD, RFC3339, a Unix timestamp, or an empty value to clear"))
+		return nil, nil, clierr.Wrap(clierr.Usage, errors.New("invalid --deadline; use YYYY-MM-DD, RFC3339 or a Unix timestamp"))
 	}
 	return &date, nil, nil
 }

internal/app/deadline_test.go

@@ -5,6 +5,7 @@ package app
 
 import (
 	"context"
+	"fmt"
 	"net/http"
 	"testing"
 
@@ -16,7 +17,7 @@ func TestDeadlineInputAndExplicitClearing(t *testing.T) {
 		for _, tc := range []struct{ value, payload string }{
 			{"2028-01-15", `{"due_date":"2028-01-15T00:00:00Z"}`},
 			{"2028-01-15T00:30:00+02:00", `{"due_date":"2028-01-15T00:30:00+02:00"}`},
-			{"01/15/2028", `{"due_date":"2028-01-15T00:00:00Z"}`},
+			{"Jan 15 2028", `{"due_date":"2028-01-15T00:00:00Z"}`},
 			{"1704067200000", `{"due_date":"2024-01-01T00:00:00Z"}`},
 			{"1704067200", `{"due_date":"2024-01-01T00:00:00Z"}`},
 			{"", `{"unset_due_date":true}`},
@@ -41,6 +42,14 @@ func TestDeadlineInputAndExplicitClearing(t *testing.T) {
 	}
 }
 
+func TestAmbiguousNumericDeadlinesAreRejected(t *testing.T) {
+	for _, value := range []string{"03/04/2026", "04/03/2026", "01/15/2028", "15.01.2028", "3/4/26"} {
+		for _, command := range [][]string{{"issue", "edit", "1"}, {"issue", "create", "--title", "x"}, {"milestone", "create", "--title", "x"}} {
+			requireUsageBeforeRequests(t, fmt.Sprintf("invalid --deadline %q: the order of day and month is ambiguous; use YYYY-MM-DD", value), append(command, "--deadline="+value)...)
+		}
+	}
+}
+
 func TestInvalidDeadlineBeforeInputCollectionOrNetwork(t *testing.T) {
 	for _, value := range []string{"bad", "2026-02-30", "0", "not-a-timestamp", "999999999999999999999999999999999999"} {
 		for _, command := range [][]string{{"issue", "edit", "1"}, {"pr", "edit", "1", "--add-reviewers=alice"}, {"issue", "create"}, {"pr", "create"}} {