@@ -28,6 +28,7 @@ import {
 } from './walk.js';
 import { resolveDriverId, type BuiltinDriverId } from '../data/driver/config-registry.zod.js';
 import { RETIRED_SUB_DAY_INTERVALS } from '../data/analytics.zod.js';
+import { ClockTimeValueSchema } from '../data/field-value.zod.js';
 import {
   FILTER_ARRAY_LOGIC_KEYWORDS,
   FILTER_OPERATORS,
@@ -7341,6 +7342,152 @@ const translationPerAppSettingsRemoved: MetadataConversion = {
   },
 };
 
+/**
+ * A `time` literal default drops a `Z` or zero-offset suffix (protocol 18).
+ *
+ * A `time` value is a zone-less wall clock (ADR-0053 D-C1), and the stored form
+ * (`ClockTimeValueSchema`) no longer admits a zone. A `Z` or a zero offset
+ * (`+00:00`, `+0000`, `-00:00`, `-0000`) names the same wall clock without it,
+ * so the suffix is dropped. A non-zero offset is left as stored and reported as
+ * a TODO: whether it meant its own digits or the UTC time is the author's call,
+ * and the parse refuses it where it lands. Only the suffixes the old stored
+ * form admitted are recognised, and `ClockTimeValueSchema` judges the rest.
+ *
+ * Reach: the literal defaults the narrowed gates judge — a `time` field's
+ * (`objects[]`, `objectExtensions[]`) and an action param typed `time`
+ * (`actions[]`, `objects[].actions[]`, an `element:button`'s inline action).
+ * Retired from the load path: an author is refused at parse, and data at rest
+ * and `os migrate meta` replay it.
+ */
+const timeDefaultUtcSuffixDropped: MetadataConversion = {
+  id: 'time-default-utc-suffix-dropped',
+  toMajor: 18,
+  retiredFromLoadPath: true,
+  retiredAfter: '17.5.0',
+  surface:
+    'object.fields.*.defaultValue / action.params[].defaultValue / '
+    + 'page.component.element:button.action.params[].defaultValue (type time)',
+  summary:
+    "a `time` literal default's `Z` or zero-offset suffix is dropped, which names the same wall "
+    + 'clock; a default with a non-zero offset is left as stored and reported as a TODO, because '
+    + 'a `time` value carries no zone (ADR-0053 D-C1) and only its author knows which wall clock '
+    + 'it meant',
+  apply(stack, emit, context) {
+    const convert = (holder: Dict, path: string, subject: string): Dict => {
+      const value = holder.defaultValue;
+      if (typeof value !== 'string') return holder;
+      const suffix = /(?:Z|[+-](?:[01]\d|2[0-3]):?[0-5]\d)$/.exec(value);
+      if (!suffix) return holder;
+      const wallClock = value.slice(0, suffix.index);
+      if (!ClockTimeValueSchema.safeParse(wallClock).success) return holder;
+      const at = `${path}.defaultValue`;
+      if (/[1-9]/.test(suffix[0])) {
+        context?.reportTodo?.({
+          path: at,
+          from: JSON.stringify(value),
+          reason: `${subject} is a time with no time zone, and this default carries a non-zero UTC `
+            + 'offset. Left as stored, it is refused where it is parsed. Rewrite it by hand as the wall '
+            + 'clock it meant, HH:MM or HH:MM:SS with no zone, or use a datetime field for an instant.',
+        });
+        return holder;
+      }
+      emit({ from: JSON.stringify(value), to: JSON.stringify(wallClock), path: at });
+      return { ...holder, defaultValue: wallClock };
+    };
+    const params = (action: Dict, path: string): Dict => {
+      const list = action.params;
+      if (!Array.isArray(list)) return action;
+      let changed = false;
+      const next = list.map((p, i) => {
+        if (!isDict(p) || p.type !== 'time') return p;
+        const name = typeof p.name === 'string' ? p.name : String(p.field ?? '');
+        const mapped = convert(p, `${path}.params[${i}]`, `Action param "${name}"`);
+        if (mapped !== p) changed = true;
+        return mapped;
+      });
+      return changed ? { ...action, params: next } : action;
+    };
+    const fields = (owner: Dict, path: string): Dict => {
+      const map = owner.fields;
+      if (!isDict(map)) return owner;
+      let changed = false;
+      const next: Dict = {};
+      for (const [name, def] of Object.entries(map)) {
+        next[name] = isDict(def) && def.type === 'time'
+          ? convert(def, `${path}.fields.${name}`, `Field "${name}"`)
+          : def;
+        if (next[name] !== def) changed = true;
+      }
+      return changed ? { ...owner, fields: next } : owner;
+    };
+    let next = mapCollection(stack, 'objects', (obj, path) =>
+      mapCollection(fields(obj, path), 'actions', (action, actionPath) => params(action, `${path}.${actionPath}`)));
+    next = mapCollection(next, 'objectExtensions', fields);
+    next = mapCollection(next, 'actions', params);
+    return mapPageComponents(next, (component, path) => {
+      if (component.type !== 'element:button') return component;
+      const properties = component.properties;
+      if (!isDict(properties) || !isDict(properties.action)) return component;
+      const action = params(properties.action, `${path}.properties.action`);
+      return action === properties.action ? component : { ...component, properties: { ...properties, action } };
+    });
+  },
+  fixture: {
+    before: {
+      objects: [{
+        name: 'shift',
+        fields: {
+          starts_at: { type: 'time', defaultValue: '09:00Z' },
+          ends_at: { type: 'time', defaultValue: '17:30:00+00:00' },
+          // A non-zero offset: left as stored (a TODO, not a notice).
+          handover_at: { type: 'time', defaultValue: '08:00+08:00' },
+          label: { type: 'text', defaultValue: '09:00Z' },
+        },
+        actions: [{ name: 'reschedule', params: [{ name: 'at', type: 'time', defaultValue: '10:00-0000' }] }],
+      }],
+      objectExtensions: [{ extend: 'shift', fields: { breaks_at: { type: 'time', defaultValue: '12:00Z' } } }],
+      actions: [{ name: 'clock_in', params: [{ name: 'at', type: 'time', defaultValue: '07:45:00.500Z' }] }],
+      pages: [{
+        name: 'shift_board',
+        regions: [{
+          name: 'main',
+          components: [{
+            type: 'element:button',
+            properties: { action: { type: 'script', target: 'clockOut', params: [{ name: 'at', type: 'time', defaultValue: '18:00Z' }] } },
+          }],
+        }],
+      }],
+    },
+    after: {
+      objects: [{
+        name: 'shift',
+        fields: {
+          starts_at: { type: 'time', defaultValue: '09:00' },
+          ends_at: { type: 'time', defaultValue: '17:30:00' },
+          handover_at: { type: 'time', defaultValue: '08:00+08:00' },
+          label: { type: 'text', defaultValue: '09:00Z' },
+        },
+        actions: [{ name: 'reschedule', params: [{ name: 'at', type: 'time', defaultValue: '10:00' }] }],
+      }],
+      objectExtensions: [{ extend: 'shift', fields: { breaks_at: { type: 'time', defaultValue: '12:00' } } }],
+      actions: [{ name: 'clock_in', params: [{ name: 'at', type: 'time', defaultValue: '07:45:00.500' }] }],
+      pages: [{
+        name: 'shift_board',
+        regions: [{
+          name: 'main',
+          components: [{
+            type: 'element:button',
+            properties: { action: { type: 'script', target: 'clockOut', params: [{ name: 'at', type: 'time', defaultValue: '18:00' }] } },
+          }],
+        }],
+      }],
+    },
+    // One per dropped suffix: two object fields, the object-nested action, the
+    // extension field, the stack action and the inline action.
+    expectedNotices: 6,
+  },
+};
+
 /**
  * `translation.pages.<name>.components.<id>.submitLabel` — the component-copy
  * key retired with its only declarer (protocol 18, commit d173125fb, ADR-0049).
@@ -12712,6 +12859,7 @@ const MAJOR_18_CONVERSIONS: readonly OrderedConversion[] = [
   { conversion: recordChatterPositionVocabulary, order: 2 },
   { conversion: recordHighlightsFieldIconRemoved, order: 10 },
   { conversion: reportJoinedChartRemoved, order: 38 },
+  { conversion: timeDefaultUtcSuffixDropped, order: 48 },
   { conversion: translationComponentSubmitLabelRemoved, order: 12 },
   { conversion: translationPerAppSettingsRemoved, order: 34 },
   { conversion: tursoConfigTimeoutToTimeoutMs, order: 28 },
 
@@ -0,0 +1,31 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+import type { SemanticMigration } from '../../types.js';
+
+// The D3 entry of the `time-default-utc-suffix-dropped` family: the stored form
+// of a `time` value (`ClockTimeValueSchema`) no longer admits a zone, so the
+// literal defaults it judges move with it.
+export const entry: SemanticMigration = {
+  id: 'time-default-zone-refused',
+  surface: 'a literal `defaultValue` with a `Z` or a UTC offset on a `time` field, or on an '
+    + 'action param typed `time`',
+  replacement: 'the wall clock itself, `HH:MM` or `HH:MM:SS` with no zone, or a `datetime` field '
+    + 'when the value is an instant. The conversion drops a `Z` or a zero offset, which names the '
+    + 'same wall clock. It does not touch a non-zero offset (`08:00+08:00`): whether that meant '
+    + '08:00 or the UTC 00:00 only the author knows, so rewrite it by hand',
+  reason:
+    'A `time` value is a zone-less wall clock (ADR-0053 D-C1), and the record validator already '
+    + 'refuses a zone-suffixed time of day on write. The stored form still admitted one, so a '
+    + 'field default such as `10:00Z` parsed clean and every insert that fell back to it was then '
+    + 'refused `invalid_time` on a field the caller never sent, and an action param default or '
+    + 'submitted value passed the dispatcher. The stored form now refuses the zone, so the field '
+    + 'and action-param default gates refuse it when it is authored and the dispatcher refuses it '
+    + 'at submit.',
+  acceptanceCriteria:
+    'No `time` field or `time` action param declares a literal default with a zone. Zone-less '
+    + 'defaults, the `NOW()` token and expression defaults parse as before. A stored `sys_metadata` '
+    + 'row whose default carried a `Z` or a zero offset loads with the zone dropped; one with a '
+    + 'non-zero offset keeps loading as stored, is listed by `os migrate meta --stored` as a TODO '
+    + 'naming the field or param, and fails the schema wherever it is parsed until it is rewritten.',
+  conversionIds: ['time-default-utc-suffix-dropped'],
+};
@@ -5808,6 +5808,18 @@ const STEP18_RATIONALE: readonly RationaleFragment[] = [
       + 'M apps is a judgment), so the entry prescribes the hand move instead of deleting '
       + 'authored content silently.',
   },
+  {
+    id: 'time-default-zone-refused',
+    order: 50,
+    text:
+      'It also narrows the `time` stored form to the zone-less wall clock the record validator '
+      + 'already enforces (ADR-0053 D-C1), so a field default or an action param default or value '
+      + 'with a `Z` or a UTC offset is refused when it is authored or submitted rather than on '
+      + 'every insert that falls back to it. The D2 conversion `time-default-utc-suffix-dropped` '
+      + 'drops a `Z` or a zero offset, which names the same wall clock, and leaves a non-zero '
+      + 'offset as stored for its author to rewrite; its D3 record is the semantic entry '
+      + '`time-default-zone-refused`.',
+  },
   {
     id: 'translation-component-submit-label-retired',
     order: 14,
@@ -16884,6 +16896,33 @@ const step18: MigrationStep = {
         + 'fails to parse with the rename prescription naming the suffixed key; the parsed defaults '
         + 'are 300 and 3600 as before.',
     },
+    // The D3 entry of the `time-default-utc-suffix-dropped` family: the stored form
+    // of a `time` value (`ClockTimeValueSchema`) no longer admits a zone, so the
+    // literal defaults it judges move with it.
+    {
+      id: 'time-default-zone-refused',
+      surface: 'a literal `defaultValue` with a `Z` or a UTC offset on a `time` field, or on an '
+        + 'action param typed `time`',
+      replacement: 'the wall clock itself, `HH:MM` or `HH:MM:SS` with no zone, or a `datetime` field '
+        + 'when the value is an instant. The conversion drops a `Z` or a zero offset, which names the '
+        + 'same wall clock. It does not touch a non-zero offset (`08:00+08:00`): whether that meant '
+        + '08:00 or the UTC 00:00 only the author knows, so rewrite it by hand',
+      reason:
+        'A `time` value is a zone-less wall clock (ADR-0053 D-C1), and the record validator already '
+        + 'refuses a zone-suffixed time of day on write. The stored form still admitted one, so a '
+        + 'field default such as `10:00Z` parsed clean and every insert that fell back to it was then '
+        + 'refused `invalid_time` on a field the caller never sent, and an action param default or '
+        + 'submitted value passed the dispatcher. The stored form now refuses the zone, so the field '
+        + 'and action-param default gates refuse it when it is authored and the dispatcher refuses it '
+        + 'at submit.',
+      acceptanceCriteria:
+        'No `time` field or `time` action param declares a literal default with a zone. Zone-less '
+        + 'defaults, the `NOW()` token and expression defaults parse as before. A stored `sys_metadata` '
+        + 'row whose default carried a `Z` or a zero offset loads with the zone dropped; one with a '
+        + 'non-zero offset keeps loading as stored, is listed by `os migrate meta --stored` as a TODO '
+        + 'naming the field or param, and fails the schema wherever it is parsed until it is rewritten.',
+      conversionIds: ['time-default-utc-suffix-dropped'],
+    },
     {
       id: 'time-update-interval-sub-day-retired',
       surface: