diff --git a/apps/desktop/src/components/generate/params/DateTimeParams.vue b/apps/desktop/src/components/generate/params/DateTimeParams.vue
index 86dca5dfb9..67fd7b0212 100644
--- a/apps/desktop/src/components/generate/params/DateTimeParams.vue
+++ b/apps/desktop/src/components/generate/params/DateTimeParams.vue
@@ -4,6 +4,7 @@ import { useI18n } from "vue-i18n";
 import { Label } from "@/components/ui/label";
 import { Button } from "@/components/ui/button";
 import { RefreshCw } from "@lucide/vue";
+import { generateTimeInRange } from "@/lib/dataGrid/generatorTimeRange";
 import type { GeneratorParams } from "@/lib/dataGrid/dataGenerate";
 
 const props = defineProps<{ params: GeneratorParams }>();
@@ -30,12 +31,7 @@ const previewVal = computed(() => {
   if (!props.params.allDay) {
     return formatLocalDate(d);
   }
-  const sh = parseInt(props.params.startTime ?? "00") || 0;
-  const sm = parseInt(props.params.endTime ?? "23") || 23;
-  const h = Math.floor(Math.random() * (sm - sh + 1)) + sh;
-  const m = Math.floor(Math.random() * 60);
-  const s = Math.floor(Math.random() * 60);
-  return `${formatLocalDate(d)} ${String(h).padStart(2, "0")}:${String(m).padStart(2, "0")}:${String(s).padStart(2, "0")}`;
+  return `${formatLocalDate(d)} ${generateTimeInRange(props.params.startTime, props.params.endTime)}`;
 });
 
 const weekdayLabels = computed(() => (["sun", "mon", "tue", "wed", "thu", "fri", "sat"] as const).map((d) => t(`dataGenerate.weekdayShort.${d}`)));
diff --git a/apps/desktop/src/components/generate/params/TimeParams.vue b/apps/desktop/src/components/generate/params/TimeParams.vue
index 8834da3eb3..3924d47a1d 100644
--- a/apps/desktop/src/components/generate/params/TimeParams.vue
+++ b/apps/desktop/src/components/generate/params/TimeParams.vue
@@ -4,6 +4,7 @@ import { useI18n } from "vue-i18n";
 import { Label } from "@/components/ui/label";
 import { Button } from "@/components/ui/button";
 import { RefreshCw } from "@lucide/vue";
+import { generateTimeInRange } from "@/lib/dataGrid/generatorTimeRange";
 import type { GeneratorParams } from "@/lib/dataGrid/dataGenerate";
 
 const props = defineProps<{ params: GeneratorParams }>();
@@ -12,17 +13,7 @@ const { t } = useI18n();
 
 const previewVal = computed(() => {
   void previewKey.value;
-  let sh = 0,
-    sm = 23;
-  if (!props.params.allDay) {
-    sh = parseInt(props.params.startTime ?? "00") || 0;
-    sm = parseInt(props.params.endTime ?? "23") || 23;
-  }
-  const h = Math.floor(Math.random() * (sm - sh + 1)) + sh;
-  const m = Math.floor(Math.random() * 60);
-  const s = Math.floor(Math.random() * 60);
-  const pad = (n: number) => String(n).padStart(2, "0");
-  return `${pad(h)}:${pad(m)}:${pad(s)}`;
+  return props.params.allDay ? generateTimeInRange() : generateTimeInRange(props.params.startTime, props.params.endTime);
 });
 
 const previewKey = ref(0);
diff --git a/apps/desktop/src/components/generate/params/__tests__/TimeRangePreview.spec.ts b/apps/desktop/src/components/generate/params/__tests__/TimeRangePreview.spec.ts
new file mode 100644
index 0000000000..acde8e0ce8
--- /dev/null
+++ b/apps/desktop/src/components/generate/params/__tests__/TimeRangePreview.spec.ts
@@ -0,0 +1,48 @@
+// @vitest-environment happy-dom
+import { createApp, h, nextTick, reactive } from "vue";
+import { createI18n } from "vue-i18n";
+import { afterEach, describe, expect, it, vi } from "vitest";
+import TimeParams from "../TimeParams.vue";
+import DateTimeParams from "../DateTimeParams.vue";
+import { generateValue, type GeneratorParams } from "@/lib/dataGrid/dataGenerate";
+
+const cleanups: Array<() => void> = [];
+afterEach(() => {
+  for (const cleanup of cleanups.splice(0)) cleanup();
+  vi.restoreAllMocks();
+});
+
+describe.each(["time", "datetime"] as const)("%s range preview", (kind) => {
+  it("matches generation and updates when minute bounds are edited", async () => {
+    vi.spyOn(Math, "random").mockReturnValue(0.5);
+    const params = reactive<GeneratorParams>({ allDay: kind === "datetime", start: "2026-10-17", end: "2026-10-17", startTime: "09:15", endTime: "09:20" });
+    const host = document.createElement("div");
+    document.body.append(host);
+    const app = createApp({ render: () => h(kind === "time" ? TimeParams : DateTimeParams, { params }) });
+    app.use(createI18n({ legacy: false, locale: "en", messages: { en: {} }, missingWarn: false, fallbackWarn: false }));
+    app.mount(host);
+    cleanups.push(() => {
+      app.unmount();
+      host.remove();
+    });
+    const generated = () => String(generateValue("t", kind, kind, 1, params));
+    expect(generated()).toContain("09:17:30");
+    expect(host.textContent).toContain(generated());
+
+    const inputs = host.querySelectorAll<HTMLInputElement>('input[type="time"]');
+    expect(inputs).toHaveLength(2);
+    for (const input of inputs) {
+      input.value = "00:00";
+      input.dispatchEvent(new Event("change", { bubbles: true }));
+    }
+    await nextTick();
+    expect(generated()).toContain("00:00:00");
+    expect(host.textContent).toContain(generated());
+    const random = vi.spyOn(Math, "random");
+    random.mockClear();
+    Array.from(host.querySelectorAll<HTMLButtonElement>("button")).at(-1)?.click();
+    await nextTick();
+    expect(random).toHaveBeenCalled();
+    expect(host.textContent).toContain(generated());
+  });
+});
diff --git a/apps/desktop/src/lib/dataGrid/__tests__/dataGenerateTime.spec.ts b/apps/desktop/src/lib/dataGrid/__tests__/dataGenerateTime.spec.ts
new file mode 100644
index 0000000000..7fdaade17b
--- /dev/null
+++ b/apps/desktop/src/lib/dataGrid/__tests__/dataGenerateTime.spec.ts
@@ -0,0 +1,38 @@
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { defaultGeneratorParams, generateValue } from "@/lib/dataGrid/dataGenerate";
+
+// Exercise the production generator, not a copy of the time-range algorithm.
+afterEach(() => vi.restoreAllMocks());
+
+describe("time generation bounds", () => {
+  it.each([
+    ["00:00", "00:00", "00:00:00", "00:00:00", "00:00:00"],
+    ["09:15", "09:20", "09:15:00", "09:17:30", "09:20:00"],
+    ["09:15:10", "09:15:12", "09:15:10", "09:15:11", "09:15:12"],
+    ["09", "09", "09:00:00", "09:30:00", "09:59:59"],
+    ["09:20", "09:15", "09:15:00", "09:17:30", "09:20:00"],
+    [undefined, undefined, "00:00:00", "12:00:00", "23:59:59"],
+    ["invalid", "25:90", "00:00:00", "12:00:00", "23:59:59"],
+  ])("honors %s..%s for time and daily datetime windows", (startTime, endTime, first, middle, last) => {
+    for (const [random, expected] of [
+      [0, first],
+      [0.5, middle],
+      [1 - Number.EPSILON, last],
+    ] as const) {
+      vi.spyOn(Math, "random").mockReturnValue(random);
+      expect(generateValue("t", "time", "time", 1, { allDay: false, startTime, endTime })).toBe(expected);
+      expect(generateValue("t", "datetime", "datetime", 1, { allDay: true, start: "2026-10-17", end: "2026-10-17", startTime, endTime })).toBe(`2026-10-17 ${expected}`);
+    }
+  });
+
+  it("ignores custom bounds in the time generator's all-day mode", () => {
+    vi.spyOn(Math, "random").mockReturnValue(0.5);
+    expect(generateValue("t", "time", "time", 1, { allDay: true, startTime: "09:15", endTime: "09:20" })).toBe("12:00:00");
+  });
+
+  it("uses valid time input values for new configurations", () => {
+    const params = defaultGeneratorParams("t", { dataType: "time", columnDefault: null }, "time");
+    expect(params.startTime).toBe("00:00");
+    expect(params.endTime).toBe("23:59");
+  });
+});
diff --git a/apps/desktop/src/lib/dataGrid/dataGenerate.ts b/apps/desktop/src/lib/dataGrid/dataGenerate.ts
index ece98d2b0a..7b6339c86c 100644
--- a/apps/desktop/src/lib/dataGrid/dataGenerate.ts
+++ b/apps/desktop/src/lib/dataGrid/dataGenerate.ts
@@ -1,5 +1,6 @@
 import type { DatabaseType } from "@/types/database";
 import { qualifiedTableName, quoteTableIdentifier } from "@/lib/table/tableSelectSql";
+import { generateTimeInRange } from "@/lib/dataGrid/generatorTimeRange";
 import { generateRegexValue } from "@/lib/dataGrid/regexGenerate";
 
 export interface GeneratorNode {
@@ -1434,8 +1435,8 @@ function buildDefaultGeneratorParams(_columnName: string, attrs: ColumnAttrs, ge
   }
 
   if (generatorKey === "time") {
-    params.startTime = "00";
-    params.endTime = "23";
+    params.startTime = "00:00";
+    params.endTime = "23:59";
     return params;
   }
 
@@ -1649,13 +1650,7 @@ export function generateValue(columnName: string, dataType: string, generatorKey
     if (params?.allDay) {
       const [rangeStart, rangeEnd] = ensureRange(start, end);
       const d = new Date(rangeStart.getTime() + Math.random() * (rangeEnd.getTime() - rangeStart.getTime()));
-      const sh = parseInt(params.startTime ?? "00") || 0;
-      const sm = parseInt(params.endTime ?? "23") || 23;
-      const h = Math.floor(Math.random() * (sm - sh + 1)) + sh;
-      const m = Math.floor(Math.random() * 60);
-      const s = Math.floor(Math.random() * 60);
-      const pad = (n: number) => String(n).padStart(2, "0");
-      return `${formatLocalDate(d)} ${pad(h)}:${pad(m)}:${pad(s)}`;
+      return `${formatLocalDate(d)} ${generateTimeInRange(params.startTime, params.endTime)}`;
     }
     return randTimestamp(start, end);
   }
@@ -1665,17 +1660,7 @@ export function generateValue(columnName: string, dataType: string, generatorKey
     return randDate(start, end);
   }
   if (key === "time") {
-    let sh = 0,
-      sm = 23;
-    if (!params?.allDay) {
-      sh = parseInt(params?.startTime ?? "00") || 0;
-      sm = parseInt(params?.endTime ?? "23") || 23;
-    }
-    const h = Math.floor(Math.random() * (sm - sh + 1)) + sh;
-    const m = Math.floor(Math.random() * 60);
-    const s = Math.floor(Math.random() * 60);
-    const pad = (n: number) => String(n).padStart(2, "0");
-    return `${pad(h)}:${pad(m)}:${pad(s)}`;
+    return params?.allDay ? generateTimeInRange() : generateTimeInRange(params?.startTime, params?.endTime);
   }
   if (key === "enum") {
     const vals = (params?.values ?? "")
diff --git a/apps/desktop/src/lib/dataGrid/generatorTimeRange.ts b/apps/desktop/src/lib/dataGrid/generatorTimeRange.ts
new file mode 100644
index 0000000000..ee4b30618f
--- /dev/null
+++ b/apps/desktop/src/lib/dataGrid/generatorTimeRange.ts
@@ -0,0 +1,23 @@
+const SECONDS_PER_DAY = 24 * 60 * 60;
+
+function timeBoundSeconds(value: string | undefined, upper: boolean): number {
+  const fallback = upper ? SECONDS_PER_DAY - 1 : 0;
+  const match = /^(\d{1,2})(?::(\d{2})(?::(\d{2}))?)?$/.exec(value?.trim() ?? "");
+  if (!match) return fallback;
+  const hour = Number(match[1]);
+  // Older saved configurations contain hours only; retain their whole-hour
+  // upper bound. Values from the time inputs are exact clock-time bounds.
+  const minute = match[2] === undefined ? (upper ? 59 : 0) : Number(match[2]);
+  const second = match[2] === undefined ? (upper ? 59 : 0) : Number(match[3] ?? 0);
+  if (hour > 23 || minute > 59 || second > 59) return fallback;
+  return hour * 3600 + minute * 60 + second;
+}
+
+export function generateTimeInRange(start?: string, end?: string): string {
+  const startSeconds = timeBoundSeconds(start, false);
+  const endSeconds = timeBoundSeconds(end, true);
+  const lower = Math.min(startSeconds, endSeconds);
+  const upper = Math.max(startSeconds, endSeconds);
+  const seconds = lower + Math.floor(Math.random() * (upper - lower + 1));
+  return [Math.floor(seconds / 3600), Math.floor((seconds % 3600) / 60), seconds % 60].map((part) => String(part).padStart(2, "0")).join(":");
+}
diff --git a/docs/assets/issue-12553/README.md b/docs/assets/issue-12553/README.md
new file mode 100644
index 0000000000..02a8d67cfd
--- /dev/null
+++ b/docs/assets/issue-12553/README.md
@@ -0,0 +1,11 @@
+# Time-range generator regression preview
+
+`time-range-preview.png` shows the production `TimeParams.vue` and
+`DateTimeParams.vue` components mounted in an isolated local Vite harness, with
+sample rows produced by the production `generateValue` function alongside them.
+It is a component preview, not a screenshot of a live database operation.
+
+The three examples cover midnight (`00:00` to `00:00`), a minute-level time
+window (`09:15` to `09:20`), and the same daily window for datetime generation.
+The automated generation and mounted-component tests cover input changes,
+refresh, endpoint selection, legacy hour-only configurations, and all-day mode.
diff --git a/docs/assets/issue-12553/time-range-preview.png b/docs/assets/issue-12553/time-range-preview.png
new file mode 100644
index 0000000000..3293904531
Binary files /dev/null and b/docs/assets/issue-12553/time-range-preview.png differ