Fastify AJV date-time schema validation missing timezone normalization

Recommended extension point

Use a global preValidation hook. AJV’s format: 'date-time' only validates the string shape; it does not transform the value, so request.body still contains a raw ISO 8601 string after schema validation. A global preValidation hook runs after validation and before the route handler, making it the correct boundary to convert matched fields to Date objects or UTC-normalized strings.

Why not schema configuration or a custom AJV format

Schema keywords describe constraints, not transformations. AJV format validators return a boolean; they are not designed to rewrite values during validation. A custom format that mutates data couples validation to conversion and makes the schema harder to test. Keep validation in the schema and conversion in the hook.

Implementation steps

1. Choose a canonical timezone, for example UTC or an IANA zone such as Europe/Paris, and declare it through an environment variable (CANONICAL_TZ) or plugin options rather than hard-coding it in every route.
2. Wrap the hook in fastify-plugin so it applies across the application without per-route duplication.
3. In the hook, convert only the fields your schema marks as format: 'date-time'. Maintain an explicit field list or inspect known body keys; do not blindly walk every string property.
4. Store the result on a new property such as request.body.normalized or replace the field only if downstream code does not need the original string.

const fp = require('fastify-plugin');
const { DateTime } = require('luxon');

module.exports = fp(async function (fastify, opts) {
  const canonicalTz = process.env.CANONICAL_TZ || 'UTC';
  const dateTimeFields = ['startsAt', 'endsAt'];

  fastify.addHook('preValidation', async (request, reply) => {
    const body = request.body;
    if (!body || typeof body !== 'object') return;

    for (const field of dateTimeFields) {
      if (typeof body[field] === 'string') {
        const parsed = DateTime.fromISO(body[field]);
        if (!parsed.isValid) continue;
        body[field] = parsed.setZone(canonicalTz).toISO();
      }
    }
  });
});

Verification

Start the server, then send a request with a non-UTC offset:

curl -X POST http://localhost:3000/bookings \
  -H 'Content-Type: application/json' \
  -d '{"startsAt":"2026-10-17T15:30:00-05:00"}'

Inspect the handler output or add a temporary log in the hook to confirm startsAt is now expressed in CANONICAL_TZ. Repeat with a different offset to verify consistent normalization.

Assumptions and uncertainty

• Assumes Fastify v4 or v5 with AJV v7 or v8 and Node.js 18+.
• Assumes Luxon or an equivalent timezone library is available; native Date cannot reliably convert between IANA zones without external data.
• Unverified: whether any existing middleware expects the raw string after validation. If yes, write the normalized value to a separate property instead of mutating request.body.

Missing diagnostic detail: Do any handlers or plugins downstream of validation require the original, unnormalized date-time string? If so, preserve it in a separate property rather than overwriting the field.