Sending UTC dates to the server using fetch

Question

I'm using fetch to send a date to the server. This should be an exact date (no time) because the server uses Java LocalDate there.

In my timezone a date like 2020-01-01 is stringified to 2019-12-31T23:00:00.000Z. The server then drops the time part. When converting to LocalDate, it doesn't know what timezone I'm in, I could be anywhere.

So I resorted to overriding Date.prototype.toJSON by a function that moves the date to UTC so that it is stringified as 2020-01-01T00:00:00.000Z. This is of course very ugly but at least it works.

Then I thought, let's override toJSON right before the request and reset it after the request returns. But whatever I tried (additional promises, timeouts), fetch uses the native toJSON. So this doesn't work:

    Date.prototype.toJSON = function () {
        return ... // Date moved to UTC. This works when set when the page loads.
    }

    const response = await fetch(...) // Fetch uses the native toJSON

    // Setting a breakpoint here reveals that toJSON is my version as expected. But fetch didn't use it.
    Date.prototype.toJSON = toJSON // Reset back to native

I read up on tasks and microtasks but found nothing that could explain this.

I also found no one with the same issue, so I'm probably going about this the wrong way.

So my questions:

• How do I send a date in any time zone to the server such that it is properly converted to a LocalDate (other than globally overriding toJSON)?
• How does fetch use the native toJSON if it is overridden right before? Just so I understand what is happening.
• The server is built using the Micronaut framework, is there something we could change there? I can't think of anything but who knows.