ui: move get_datetime tool to frontend (#27255)

* ui: move get_datetime tool to frontend

* clarify docs

* server: drop the now unused ctime include

strftime() and gmtime_r() were the only users, both went away with the
get_datetime tool. Also make the renderer's catch inert: the browser
executor always emits JSON, so a non-JSON result is no longer a date to
display.

---------

Co-authored-by: Pascal <admin@serveurperso.com>

tools/ui/src/lib/utils/get-datetime.ts

@@ -0,0 +1,38 @@
+/**
+ * Frontend executor for the `get_datetime` tool. It runs in the browser, so it
+ * reports the user's own clock and time zone instead of the server's UTC time -
+ * a chat about "tomorrow" means the user's tomorrow, not the host's.
+ *
+ * @see buildGetDatetimeToolDefinition in constants/get-datetime.ts - tool schema sent to the LLM
+ */
+
+import type { ToolExecutionResult } from '$lib/types';
+
+function pad(value: number): string {
+	return String(value).padStart(2, '0');
+}
+
+/** ISO 8601 in local time, e.g. `2026-08-17T14:05:09+02:00` */
+function localIsoString(date: Date): string {
+	// getTimezoneOffset() counts minutes behind UTC, ISO 8601 counts them ahead
+	const offset = -date.getTimezoneOffset();
+	const sign = offset < 0 ? '-' : '+';
+	const absOffset = Math.abs(offset);
+	const day = `${date.getFullYear()}-${pad(date.getMonth() + 1)}-${pad(date.getDate())}`;
+	const time = `${pad(date.getHours())}:${pad(date.getMinutes())}:${pad(date.getSeconds())}`;
+
+	return `${day}T${time}${sign}${pad(Math.floor(absOffset / 60))}:${pad(absOffset % 60)}`;
+}
+
+/** The `result` field keeps the shape the `get_datetime` renderer already reads. */
+export function executeGetDatetimeTool(): ToolExecutionResult {
+	const now = new Date();
+
+	return {
+		content: JSON.stringify({
+			result: localIsoString(now),
+			timezone: Intl.DateTimeFormat().resolvedOptions().timeZone
+		}),
+		isError: false
+	};
+}

tools/ui/src/lib/utils/index.ts

@@ -331,6 +331,9 @@ export { getChatCommands } from './chat-commands';
 // SANDBOX_TOOL_DEFINITION is deprecated; kept for backward compatibility.
 export { buildSandboxToolDefinition, SANDBOX_TOOL_DEFINITION } from './sandbox-tool';
 
+// Frontend `get_datetime` executor (the browser clock, not the server's)
+export { executeGetDatetimeTool } from './get-datetime';
+
 // Browser fallback for the server's get_info tool
 export { executeBrowserInfoTool } from './browser-info';