🐛 fix: normalize AI-generated dates to valid ISO offset datetimes

- add normalizeAiDateString() to coerce bare ISO datetimes, date-only
  strings, and fractional-second variants into offset-aware format
- apply via z.preprocess in AiEventResponseItemSchema so Zod validation
  no longer rejects AI responses missing a timezone offset
- fix system prompt to use toISOString() (unambiguous UTC) and clarify
  expected datetime format for the AI model
- install bun-types and add to tsconfig so bun:test resolves cleanly
- add 8 behaviour-driven tests covering all normalizer edge cases

src/app/api/ai-event/route.ts

@@ -17,7 +17,7 @@ TypeScript type:
   description?: string,
   location?: string,
   url?: string,
-  start: string, // ISO datetime
+  start: string, // ISO 8601 datetime with timezone, e.g. "2025-04-10T10:00:00-04:00"
   end?: string,
   allDay?: boolean,
   recurrenceRule?: string // valid iCal RRULE string like FREQ=WEEKLY;BYDAY=MO;INTERVAL=1
@@ -26,7 +26,7 @@ TypeScript type:
 Rules:
   - If the user describes multiple events in one prompt, return multiple objects (one per event).
   - Always return a valid JSON array of objects, even if there's only one event.
-  - Today is ${new Date().toLocaleString()}.
+  - Today is ${new Date().toISOString()}.
   - If no time is given, assume allDay event.
   - If no end time is given (and event is not allDay), default to 1 hour after start.
   - If multiple events are described, return multiple.

src/lib/date-normalizer.ts

@@ -0,0 +1,10 @@
+import { formatISO, parseISO } from "date-fns";
+
+const ISO_DATETIME_WITH_OFFSET =
+	/^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}(Z|[+-]\d{2}:\d{2})$/;
+
+export const normalizeAiDateString = (input: string): string => {
+	if (ISO_DATETIME_WITH_OFFSET.test(input)) return input;
+	const parsed = parseISO(input);
+	return formatISO(parsed);
+};

src/lib/types.ts

@@ -1,5 +1,6 @@
 import { z } from "zod";
 import { MAX_IMAGE_SIZE_BYTES } from "@/lib/constants";
+import { normalizeAiDateString } from "@/lib/date-normalizer";
 
 /** Validates that a base64 data URL string decodes to binary under the max size. */
 const isValidImageSize = (val: string | undefined): boolean => {
@@ -29,14 +30,19 @@ export const AiEventRequestSchema = z
 
 export type AiEventRequest = z.infer<typeof AiEventRequestSchema>;
 
+const aiDatetime = z.preprocess(
+	(val) => (typeof val === "string" ? normalizeAiDateString(val) : val),
+	z.string().datetime({ offset: true }),
+);
+
 export const AiEventResponseItemSchema = z.object({
 	id: z.string().optional(),
 	title: z.string().min(1),
 	description: z.string().optional(),
 	location: z.string().optional(),
 	url: z.string().optional(),
-	start: z.string().datetime({ offset: true }),
-	end: z.string().datetime({ offset: true }).optional(),
+	start: aiDatetime,
+	end: aiDatetime.optional(),
 	allDay: z.boolean().optional(),
 	recurrenceRule: z.string().optional(),
 });