diff --git a/vueManager/package.json b/vueManager/package.json
index 370f116e..a28cd519 100644
--- a/vueManager/package.json
+++ b/vueManager/package.json
@@ -12,7 +12,8 @@
     "format": "prettier --write \"src/**/*.{js,vue,scss,css}\"",
     "format:check": "prettier --check \"src/**/*.{js,vue,scss,css}\"",
     "lint:all": "npm run lint && npm run format:check && npm run lint:scss",
-    "lint:scss": "stylelint \"src/**/*.scss\" --fix"
+    "lint:scss": "stylelint \"src/**/*.scss\" --fix",
+    "test:utils": "node src/utils/formatLocalDateYmd.test.mjs"
   },
   "dependencies": {
     "@primeuix/themes": "^2.0.3",
diff --git a/vueManager/src/components/OrdersGrid.vue b/vueManager/src/components/OrdersGrid.vue
index cbae3248..fdfb8301 100644
--- a/vueManager/src/components/OrdersGrid.vue
+++ b/vueManager/src/components/OrdersGrid.vue
@@ -17,6 +17,7 @@ import { computed, onMounted, ref } from 'vue'
 import { useGridFilterParams } from '../composables/useGridFilterParams.js'
 import { useSelection } from '../composables/useSelection.js'
 import request from '../request.js'
+import { formatLocalDateYmd } from '../utils/formatLocalDateYmd.js'
 import ActionsColumn from './ActionsColumn.vue'
 
 const toast = useToast()
@@ -106,18 +107,18 @@ async function loadOrders() {
             addFilterParam(
               params,
               filterConfig.fields?.from || `${key}_from`,
-              formatDateForApi(value[0])
+              formatLocalDateYmd(value[0])
             )
           }
           if (value[1]) {
             addFilterParam(
               params,
               filterConfig.fields?.to || `${key}_to`,
-              formatDateForApi(value[1])
+              formatLocalDateYmd(value[1])
             )
           }
         } else if (filterConfig?.type === 'datepicker' && value) {
-          addFilterParam(params, key, formatDateForApi(value))
+          addFilterParam(params, key, formatLocalDateYmd(value))
         } else {
           addFilterParam(params, key, value)
         }
@@ -169,15 +170,6 @@ function onSort(event) {
   loadOrders()
 }
 
-/**
- * Format date for API (YYYY-MM-DD)
- */
-function formatDateForApi(date) {
-  if (!date) return null
-  const d = new Date(date)
-  return d.toISOString().split('T')[0]
-}
-
 /**
  * Open order for editing (redirect to order page)
  */
diff --git a/vueManager/src/components/product/ProductOptionField.vue b/vueManager/src/components/product/ProductOptionField.vue
index 7865cebd..cf43cbf5 100644
--- a/vueManager/src/components/product/ProductOptionField.vue
+++ b/vueManager/src/components/product/ProductOptionField.vue
@@ -11,6 +11,7 @@ import Textarea from 'primevue/textarea'
 import { computed, ref, watch } from 'vue'
 
 import request from '../../request.js'
+import { formatLocalDateYmd } from '../../utils/formatLocalDateYmd.js'
 
 const props = defineProps({
   option: { type: Object, required: true },
@@ -80,17 +81,9 @@ function onChange() {
   emit('change', { key: props.option.key, value: value.value })
 }
 
-/**
- * Format a Date in local timezone as YYYY-MM-DD.
- * Never use Date.toISOString() here — it converts to UTC and shifts the date east of UTC
- * (user picks 2026-04-20, toISOString() returns 2026-04-19).
- */
 function formatDateForPost(raw) {
   if (raw instanceof Date) {
-    const y = raw.getFullYear()
-    const m = String(raw.getMonth() + 1).padStart(2, '0')
-    const d = String(raw.getDate()).padStart(2, '0')
-    return `${y}-${m}-${d}`
+    return formatLocalDateYmd(raw) || ''
   }
   return raw || ''
 }
diff --git a/vueManager/src/utils/formatLocalDateYmd.js b/vueManager/src/utils/formatLocalDateYmd.js
new file mode 100644
index 00000000..64b0517a
--- /dev/null
+++ b/vueManager/src/utils/formatLocalDateYmd.js
@@ -0,0 +1,24 @@
+/**
+ * Format a date as YYYY-MM-DD in the browser's local calendar.
+ *
+ * Do not use Date.toISOString().split('T')[0] — ISO is UTC and shifts the
+ * calendar day east/west of UTC (issue #386).
+ *
+ * @param {string|number|Date|null|undefined} date
+ * @returns {string|null}
+ */
+export function formatLocalDateYmd(date) {
+  if (date === null || date === undefined || date === '') {
+    return null
+  }
+
+  const d = date instanceof Date ? date : new Date(date)
+  if (Number.isNaN(d.getTime())) {
+    return null
+  }
+
+  const y = d.getFullYear()
+  const m = String(d.getMonth() + 1).padStart(2, '0')
+  const day = String(d.getDate()).padStart(2, '0')
+  return `${y}-${m}-${day}`
+}
diff --git a/vueManager/src/utils/formatLocalDateYmd.test.mjs b/vueManager/src/utils/formatLocalDateYmd.test.mjs
new file mode 100644
index 00000000..d4b75886
--- /dev/null
+++ b/vueManager/src/utils/formatLocalDateYmd.test.mjs
@@ -0,0 +1,40 @@
+/**
+ * Regression: local YYYY-MM-DD must not use UTC from toISOString (#386).
+ *
+ * Run: node src/utils/formatLocalDateYmd.test.mjs
+ */
+
+import assert from 'node:assert/strict'
+import { stdout } from 'node:process'
+
+import { formatLocalDateYmd } from './formatLocalDateYmd.js'
+
+assert.equal(formatLocalDateYmd(null), null)
+assert.equal(formatLocalDateYmd(''), null)
+assert.equal(formatLocalDateYmd('invalid'), null)
+
+// Local calendar components (month is 0-based)
+const localMidnight = new Date(2026, 3, 20, 0, 0, 0)
+assert.equal(formatLocalDateYmd(localMidnight), '2026-04-20')
+
+// Early local morning east of UTC: toISOString often yields previous UTC day
+const earlyLocalMorning = new Date(2026, 3, 21, 1, 0, 0)
+assert.equal(formatLocalDateYmd(earlyLocalMorning), '2026-04-21')
+
+const utcDay = earlyLocalMorning.toISOString().split('T')[0]
+const offsetMinutes = earlyLocalMorning.getTimezoneOffset()
+if (offsetMinutes < 0 && utcDay !== '2026-04-21') {
+  // Negative getTimezoneOffset => local is ahead of UTC (e.g. Asia/Almaty)
+  assert.notEqual(
+    utcDay,
+    formatLocalDateYmd(earlyLocalMorning),
+    'toISOString must differ from local YMD east of UTC (documents #386 bug)'
+  )
+}
+
+assert.equal(
+  formatLocalDateYmd('2026-04-20T12:00:00'),
+  formatLocalDateYmd(new Date('2026-04-20T12:00:00'))
+)
+
+stdout.write('OK formatLocalDateYmd.test.mjs\n')