Worker global variable mystery

I tried to use a global variable in my worker:

const date1 = new Date();
export default {
  async fetch(request) {
    const date2 = new Date();
    return new Response("ok");

Why is date1 always:

Thu Jan 01 1970 00:00:00 GMT+0000 (Coordinated Universal Time)

while date2 contains the correct current date?