docs: add troubleshooting guide for datetime timezone issue

Add comprehensive documentation for handling datetime timezone discrepancies between backend and frontend. The issue arises when backend returns naive datetime strings without timezone suffixes, causing JavaScript to parse them as local time instead of UTC. The guide includes problem description, root cause analysis, three frontend solutions (dayjs UTC parsing, manual 'Z' suffix addition, and global Axios interceptor), backend context, and testing checklist.

docs/gns3-copilot/troubleshooting/datetime-timezone-issue.md

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+# Datetime Timezone Issue
+
+## Problem Description
+
+The API returns datetime strings without timezone suffixes, causing frontend JavaScript to parse them as local time instead of UTC time.
+
+### Example
+
+| Aspect | Value |
+|--------|-------|
+| Actual time (Beijing) | 2026-03-07 01:30 |
+| Backend returns | `2026-03-06T16:31:36.547762` (no timezone) |
+| Frontend displays | March 6, 16:31 |
+| Should display | March 7, 00:31 (UTC 16:31 + 8 hours) |
+
+### Root Cause
+
+- Backend stores UTC time in database as naive datetime (no timezone info)
+- FastAPI's `jsonable_encoder` serializes datetime as ISO 8601 **without timezone suffix**
+- JavaScript `new Date()` treats strings without 'Z' or timezone offset as **local time**
+
+```
+Backend intention: 2026-03-06T16:31:36Z (UTC)
+Actually returned: 2026-03-06T16:31:36.547762 (no timezone)
+JavaScript parses as: local time 16:31
+Should be: UTC 16:31 → Beijing time 00:31 (next day)
+```
+
+---
+
+## Frontend Solutions
+
+### Solution 1: Use dayjs UTC Parsing (Recommended)
+
+```typescript
+import dayjs from 'dayjs';
+import utc from 'dayjs/plugin/utc';
+import timezone from 'dayjs/plugin/timezone';
+
+dayjs.extend(utc);
+dayjs.extend(timezone);
+
+// Parse naive datetime as UTC
+const createdAt = '2026-03-06T16:31:36.547762';
+const date = dayjs.utc(createdAt).tz('Asia/Shanghai');
+console.log(date.format('YYYY-MM-DD HH:mm')); // 2026-03-07 00:31
+```
+
+### Solution 2: Add 'Z' Suffix Manually
+
+```typescript
+// Utility function to normalize API dates
+function parseAPIDate(dateStr: string): Date {
+  // Add 'Z' suffix if no timezone info present
+  const normalized = dateStr.includes('Z') || dateStr.includes('+')
+    ? dateStr
+    : `${dateStr}Z`;
+  return new Date(normalized);
+}
+
+// Usage
+const date = parseAPIDate('2026-03-06T16:31:36.547762');
+console.log(date.toLocaleString('zh-CN', { timeZone: 'Asia/Shanghai' }));
+```
+
+### Solution 3: Global Axios Interceptor (Most Thorough)
+
+```typescript
+import axios from 'axios';
+
+api.interceptors.response.use((response) => {
+  // Recursively normalize all date fields
+  function normalizeDates(obj: any): any {
+    // Match ISO 8601 datetime pattern without timezone
+    if (typeof obj === 'string' && /^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}/.test(obj)) {
+      return obj.includes('Z') || obj.includes('+') ? obj : `${obj}Z`;
+    }
+    if (Array.isArray(obj)) {
+      return obj.map(normalizeDates);
+    }
+    if (obj && typeof obj === 'object') {
+      return Object.fromEntries(
+        Object.entries(obj).map(([k, v]) => [k, normalizeDates(v)])
+      );
+    }
+    return obj;
+  }
+
+  response.data = normalizeDates(response.data);
+  return response;
+});
+```
+
+---
+
+## Backend Context
+
+### Current Implementation
+
+**File**: `gns3server/api/server.py:58-64`
+
+```python
+application = FastAPI(
+    title="GNS3 controller API",
+    description="This page describes the public controller API for GNS3",
+    version="v3",
+    docs_url=None,
+    redoc_url=None
+)
+```
+
+**File**: `gns3server/db/models/base.py`
+
+```python
+from fastapi.encoders import jsonable_encoder
+
+class Base:
+    def asjson(self):
+        return jsonable_encoder(self.asdict())
+```
+
+### Datetime Flow
+
+1. Database stores naive datetime (no timezone)
+2. FastAPI uses `jsonable_encoder` to serialize
+3. Output format: `YYYY-MM-DDTHH:MM:SS.ffffff` (no 'Z' suffix)
+4. JavaScript interprets as local time
+
+### Affected Fields
+
+- `created_at`
+- `updated_at`
+- `last_login`
+- Any other datetime fields in API responses
+
+---
+
+## Testing Checklist
+
+- [ ] Verify datetime displays correctly in Beijing timezone (UTC+8)
+- [ ] Test with other timezones (e.g., UTC-5, UTC+0)
+- [ ] Check daylight saving time transitions (if applicable)
+- [ ] Verify datetime input/insertion still works correctly
+
+---
+
+## Related Files
+
+- Backend: `gns3server/api/server.py`
+- Backend: `gns3server/db/models/base.py`
+- Schemas: `gns3server/schemas/controller/base.py`