Fixed some tests.

internal/app/date_time_test.go

@@ -3,6 +3,8 @@ package app
 import (
 	"context"
 	"log/slog"
+	"os"
+	"os/exec"
 	"path/filepath"
 	"testing"
 	"time"
@@ -14,6 +16,18 @@ import (
 )
 
 func TestDateTimeFormats(t *testing.T) {
+	// datetime initializes libc's locale before tests run. Use a child process
+	// so these fixed English expectations do not depend on the build locale.
+	if os.Getenv("CTHULHU_DATE_TIME_TEST_CHILD") != "1" {
+		ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+		defer cancel()
+		command := exec.CommandContext(ctx, os.Args[0], "-test.run=^TestDateTimeFormats$", "-test.count=1")
+		command.Env = append(os.Environ(), "LC_ALL=C", "CTHULHU_DATE_TIME_TEST_CHILD=1")
+		if output, err := command.CombinedOutput(); err != nil {
+			t.Fatalf("C-locale date/time test: %v\n%s", err, output)
+		}
+		return
+	}
 	now := time.Date(2026, 9, 17, 19, 5, 0, 0, time.UTC)
 	for _, tc := range []struct {
 		command      input.Command