diff --git a/api/dashboard_charts.php b/api/dashboard_charts.php
index 6a09946..a5e8b71 100644
--- a/api/dashboard_charts.php
+++ b/api/dashboard_charts.php
@@ -37,11 +37,11 @@
 /*    tek eksende gosterilebilir).                                      */
 /* ------------------------------------------------------------------ */
 
-$months = [];
-for ($i = 11; $i >= 0; $i--) {
-    $months[date('Y-m', strtotime("-{$i} month"))] = ['opened' => 0, 'closed' => 0];
-}
-$since = date('Y-m-01', strtotime('-11 month'));
+// Takvim ayi penceresi ayin 1'ine sabitlenerek kurulur (bkz. recent_months):
+// strtotime('-N month') ay sonu gunlerinde bir sonraki aya tasiyordu.
+$monthKeys = recent_months(12);
+$months = array_fill_keys($monthKeys, ['opened' => 0, 'closed' => 0]);
+$since = $monthKeys[0] . '-01';
 
 $rows = $pdo->prepare(
     "SELECT DATE_FORMAT(created_at, '%Y-%m') AS ay, COUNT(*) AS adet
diff --git a/includes/functions.php b/includes/functions.php
index 92f9613..b0a854b 100644
--- a/includes/functions.php
+++ b/includes/functions.php
@@ -216,6 +216,36 @@ function days_until(?string $date): ?int
     return (int)floor(($t - strtotime(date('Y-m-d'))) / 86400);
 }
 
+/**
+ * Son $count takvim ayinin 'Y-m' anahtarlarini eskiden yeniye sirali dondurur.
+ * Pencerenin alt siniri (SQL :since degeri) ilk anahtarin '-01' ekidir.
+ *
+ * NEDEN AYRI BIR FONKSIYON: strtotime('-N month') gun tasmasini kirmaz;
+ * ayin 29-31'inde hedef ayda o gun yoksa sonuc bir SONRAKI aya taser.
+ * Dashboard trend penceresi (api/dashboard_charts.php) bu yuzden ay sonu
+ * isteklerinde 12 yerine 7-11 ay uretiyor, kayip aylarin riskleri hic
+ * sayilmiyordu. Burada ay aritmetigi her zaman ayin 1'ine sabitlenir;
+ * 1. gunden cikarilan ay asla tasmaz.
+ *
+ * @param int $count Kac ay dondurecek (1 = yalnizca bulunulan ay).
+ * @param int|null $baseTs Pencerenin bittigi an (null = simdi).
+ * @return list<string> 'YYYY-MM' anahtarlari, eskiden yeniye.
+ */
+function recent_months(int $count, ?int $baseTs = null): array
+{
+    if ($count < 1) {
+        return [];
+    }
+    $anchor = (new DateTimeImmutable(date('Y-m-d H:i:s', $baseTs ?? time())))
+        ->modify('first day of this month midnight');
+
+    $out = [];
+    for ($i = $count - 1; $i >= 0; $i--) {
+        $out[] = $anchor->modify("-{$i} months")->format('Y-m');
+    }
+    return $out;
+}
+
 function str_limit(?string $text, int $limit = 80): string
 {
     $text = trim((string)$text);
diff --git a/reports/executive_summary.php b/reports/executive_summary.php
index f23ee2d..b18584e 100644
--- a/reports/executive_summary.php
+++ b/reports/executive_summary.php
@@ -85,12 +85,12 @@
      LIMIT 8"
 )->fetchAll();
 
-/* Son 6 ay: açılan ve kapanan */
-$trend = [];
-for ($i = 5; $i >= 0; $i--) {
-    $trend[date('Y-m', strtotime("-{$i} month"))] = ['acilan' => 0, 'kapanan' => 0];
-}
-$since = date('Y-m-01', strtotime('-5 month'));
+/* Son 6 ay: açılan ve kapanan — takvim ayı penceresi ayın 1'ine
+   sabitlenerek kurulur (bkz. recent_months): strtotime('-N month')
+   ay sonu günlerinde bir sonraki aya taşıyordu. */
+$monthKeys = recent_months(6);
+$trend = array_fill_keys($monthKeys, ['acilan' => 0, 'kapanan' => 0]);
+$since = $monthKeys[0] . '-01';
 
 $q = db()->prepare("SELECT DATE_FORMAT(created_at,'%Y-%m') AS ay, COUNT(*) AS n
                     FROM risks WHERE deleted_at IS NULL AND created_at >= :s GROUP BY ay");
diff --git a/tools/trend_months_test.php b/tools/trend_months_test.php
new file mode 100644
index 0000000..5ea9d5a
--- /dev/null
+++ b/tools/trend_months_test.php
@@ -0,0 +1,147 @@
+<?php
+declare(strict_types=1);
+
+/**
+ * RiskOps - Ay bazli trend pencereleri regresyon testi
+ * Calistirma:  php tools/trend_months_test.php
+ *
+ * Bagimlilik YOK: veritabani ve bootstrap gerekmez, yalnizca
+ * includes/functions.php icindeki recent_months() test edilir.
+ *
+ * Kapsanan tuketiciler:
+ *   api/dashboard_charts.php      (12 aylik trend penceresi)
+ *   reports/executive_summary.php (6 aylik "Son 6 Ay" penceresi)
+ *
+ * REGRESYON ARKAPLANI: pencereler bir zamanlar
+ *     date('Y-m', strtotime("-{$i} month"))
+ * ile kuruluyordu. strtotime ay aritmetiginde gun tasmasini kirmaz;
+ * ayin 29-31'inde hedef ayda o gun yoksa sonuc bir SONRAKI aya tasiyor,
+ * ayni 'Y-m' anahtari iki kez uretiliyor ve pencere kuculuyordu
+ * (dashboard 7-11 aya, yonetici ozeti 3-5 aya dusuyordu; kayip
+ * aylarin riskleri hic sayilmiyordu).
+ */
+
+if (PHP_SAPI !== 'cli') {
+    http_response_code(403);
+    exit('CLI only.');
+}
+
+require_once __DIR__ . '/../includes/functions.php';
+
+$PASS = 0;
+$FAIL = 0;
+
+function check(string $label, bool $ok, string $detail = ''): void
+{
+    global $PASS, $FAIL;
+    if ($ok) {
+        $PASS++;
+        printf("  [ OK ]  %-52s %s\n", $label, $detail);
+    } else {
+        $FAIL++;
+        printf("  [FAIL]  %-52s %s\n", $label, $detail);
+    }
+}
+
+function section(string $title): void
+{
+    echo "\n" . str_repeat('-', 72) . "\n  " . $title . "\n" . str_repeat('-', 72) . "\n";
+}
+
+/**
+ * ORACLE — beklenen ay listesi mktime() ay normalizasyonuyla hesaplanir
+ * (recent_months icindeki kodun bagimsizi).
+ */
+function expected_months(int $baseTs, int $count): array
+{
+    [$y, $m] = array_map('intval', explode('-', date('Y-m', $baseTs)));
+    $out = [];
+    for ($k = $count - 1; $k >= 0; $k--) {
+        $out[] = date('Y-m', mktime(1, 1, 1, $m - $k, 1, $y));
+    }
+    return $out;
+}
+
+echo "\n=============== Trend Month Window Regression Test ===============";
+
+/* ------------------------------------------------------------------ */
+section('1. Ay sonu gunlerinde pencere (regresyon)');
+/* ------------------------------------------------------------------ */
+
+// Hatali davranisin kanitlandigi gunler: 29-31 (hedef ay kisa ise tasma).
+$probes = [
+    '2026-05-31', '2026-03-31', '2026-08-31', '2025-12-31',
+    '2026-01-31', '2026-01-30', '2024-02-29', '2026-02-28',
+    '2026-10-31', '2026-04-30', '2025-11-30', '2026-12-31',
+];
+
+foreach ($probes as $d) {
+    $bts = strtotime($d . ' 12:00:00');
+    $got = recent_months(12, $bts);
+    check(
+        "12 ay: {$d}",
+        $got === expected_months($bts, 12) && count(array_unique($got)) === 12,
+        count($got) . ' ay'
+    );
+}
+
+/* ------------------------------------------------------------------ */
+section('2. Sinir ve ikincil dallar');
+/* ------------------------------------------------------------------ */
+
+$now = time();
+check('bugun (varsayilan taban): 12 ay',
+    recent_months(12) === expected_months($now, 12));
+check('taban null acik gecilirse ayni sonuc',
+    recent_months(12, null) === recent_months(12));
+check('count=1 yalnizca bulunulan ay',
+    recent_months(1, $now) === [date('Y-m', $now)]);
+check('count=0 bos liste', recent_months(0, $now) === []);
+check('count negatif bos liste', recent_months(-3, $now) === []);
+check('count=13 yil sinirini asar',
+    recent_months(13, strtotime('2026-01-15 12:00:00'))
+        === expected_months(strtotime('2026-01-15 12:00:00'), 13));
+check('count=6 yonetici ozeti penceresi (2026-05-31 taban)',
+    recent_months(6, strtotime('2026-05-31 12:00:00'))
+        === expected_months(strtotime('2026-05-31 12:00:00'), 6));
+check('anahtarlar eskiden yeniye sirali',
+    recent_months(12, $now) === array_values(array_sort(recent_months(12, $now))));
+
+/* SQL :since degeri: en eski ayin 1'i (api/dashboard_charts.php tuketimi) */
+$keys = recent_months(12, strtotime('2026-05-31 12:00:00'));
+check('since = en eski ayin 1\'i (2026-05-31 taban)',
+    $keys[0] . '-01' === '2025-06-01', $keys[0] . '-01');
+
+/* ------------------------------------------------------------------ */
+section('3. Uc nokta baglantisi');
+/* ------------------------------------------------------------------ */
+
+$src = (string)file_get_contents(__DIR__ . '/../api/dashboard_charts.php');
+check('api/dashboard_charts.php recent_months() kullaniyor',
+    str_contains($src, 'recent_months(12)'));
+check('dashboard bozuk strtotime("-N month") ifadesi kaldirildi',
+    !str_contains($src, 'strtotime("-{'));
+
+$srcExec = (string)file_get_contents(__DIR__ . '/../reports/executive_summary.php');
+check('reports/executive_summary.php recent_months() kullaniyor',
+    str_contains($srcExec, 'recent_months(6)'));
+check('yonetici ozeti bozuk strtotime ay ifadeleri kaldirildi',
+    !str_contains($srcExec, 'strtotime("-{') && !str_contains($srcExec, "strtotime('-5 month')"));
+
+/* ------------------------------------------------------------------ */
+
+echo "\n" . str_repeat('-', 72) . "\n";
+printf("Sonuc: %d OK, %d FAIL\n", $PASS, $FAIL);
+if ($FAIL > 0) {
+    echo "TREND MONTH WINDOW TEST: FAIL\n";
+    exit(1);
+}
+echo "TREND MONTH WINDOW TEST: PASS\n";
+exit(0);
+
+/** Kucukten buyuge siralar (strcmp) — 'Y-m' anahtarlari icin yeterli. */
+function array_sort(array $a): array
+{
+    usort($a, static fn(string $x, string $y) => strcmp($x, $y));
+    return $a;
+}