fix: use ISO 8601 timestamp with timezone offset in system prompt

Replace imprecise toDateString() (e.g. 'Mon Feb 23 2026') with full
ISO 8601 UTC timestamp and user timezone with UTC offset when the
extension provides it via editorContext.timezone.

Before: Today's date: Mon Feb 23 2026
After:  Current time: 2026-02-23T12:23:04.159Z
        User timezone: Europe/Amsterdam, UTC+1:00

packages/opencode/src/session/system.ts

@@ -36,6 +36,21 @@ export namespace SystemPrompt {
     return [PROMPT_ANTHROPIC_WITHOUT_TODO]
   }
 
+  // kilocode_change start
+  function formatTime(timezone?: string): string[] {
+    const now = new Date()
+    const lines = [`  Current time: ${now.toISOString()}`]
+    if (timezone) {
+      const offset = -now.getTimezoneOffset()
+      const sign = offset >= 0 ? "+" : "-"
+      const hours = Math.floor(Math.abs(offset) / 60)
+      const mins = Math.abs(offset) % 60
+      lines.push(`  User timezone: ${timezone}, UTC${sign}${hours}:${mins.toString().padStart(2, "0")}`)
+    }
+    return lines
+  }
+  // kilocode_change end
+
   export async function environment(
     model: Provider.Model,
     editorContext?: {
@@ -51,14 +66,11 @@ export namespace SystemPrompt {
       `  Working directory: ${Instance.directory}`,
       `  Is directory a git repo: ${project.vcs === "git" ? "yes" : "no"}`,
       `  Platform: ${process.platform}`,
-      `  Today's date: ${new Date().toDateString()}`,
+      ...formatTime(editorContext?.timezone), // kilocode_change
     ]
     if (editorContext?.shell) {
       envLines.push(`  Default shell: ${editorContext.shell}`)
     }
-    if (editorContext?.timezone) {
-      envLines.push(`  Timezone: ${editorContext.timezone}`)
-    }
     if (editorContext?.activeFile) {
       envLines.push(`  Active file: ${editorContext.activeFile}`)
     }