Joi conditional validation on two dates

Question

I would like to validate against two dates in an object. The rules should be:

• a must include one valid date, either b or c must be present and valid
• If b or c is null it should not validate that value...if the other value is valid.

It should validate these:

{
  a: {
    b: '2021-11-01T00:00:00.000Z'
    c: null
  }
}

{
  a: {
    b: '2021-11-01T00:00:00.000Z',
    c: '2021-11-01T00:00:00.000Z'
  }
}

{
  a: {
    b: null,
    c: '2021-11-01T00:00:00.000Z'
  }
}

It should NOT validate these:

{
  a: {
    b: null
    c: null
  }
}

I was thinking of using an or but it seems when the value is null it doesn't check. The reason for the null is that the calendar component I'm using sets it to null if the user clears the date picker.

{
  a: Joi.object({
    b: Joi.date(),
    c: Joi.date(),
  }).or('b', 'c'),
}