I have a ReactJs project where also i use jest for testing. To display the time according UK timezone, i created this function:

 const getDateTZ = dt => {
  const d = new Date(dt)
  const o = {
    hour: '2-digit',
    minute: '2-digit',
    timeZone: 'Europe/London'
  }
  return d.toLocaleTimeString('en-GB', o)
}
console.log(getDateTZ(new Date()))

This function work fine and running it i get the right time. To test this function with jest i add this:

test('It should return time', () => {
  const today = new Date()
  today.setHours(11)
  today.setMinutes(30)
  const r = getDateTZ(today)
  expect(r).toEqual('09:30')
})

Expected: "09:30"
Received: "11:30"

So when i run all the tests my test fails because i receive 11:30 instead of 09:30 even if the test works if i run it separately in the test file.
Question: Why i get this issue and how to get rid of it?