Fixed PublishDateTimePicker issue with Now() returning null

[Mayank-Tripathi32]

Resolves #60281

What?

This PR fixes an issue in the publish date-time picker where clicking the "Now" button was returning null instead of the current datetime, causing incorrect GMT date settings when user switches to next day and back.

Why?

Currently, when users click the "Now" button in the publish date-time picker, the component returns null, which creates an inconsistency with the DatePickerComponent's expected behavior. This results in dates being incorrectly set in GMT format, impacting the accuracy of publish timestamps.

The change is necessary to ensure consistent datetime handling across the application and prevent intermittent parsing issues between component updates.

How?

The implementation modifies the "Now" button click handler to return the current datetime as an ISO string (without timezone information) instead of null. This aligns with the expected input format for both TimePicker and DateTimePicker components.
The change replaces:

onClick: () => onChange?.( null )

with:

const handleNowClick = () => {
    const now = new Date().toISOString().slice(0, 19);
    onChange?.(now);
};

Testing Instructions

1. Open the post editor
2. Click on the Post Settings sidebar (gear icon)
3. Locate the Publish section
4. Click on the date/time picker to open it
   5.1 Click on future Date then observe that publish has changed to schedule
   5.2 Click the "Now" button in the picker header and observe the publish button
   5.3 Click on future date and then switch to current date manually,
5. Observe that schedule is changed to publish as expected. (Required behaviour)
6. Save the post and verify the publish timestamp is accurate

Screencast

Test.Publish.mp4

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Co-authored-by: Mayank-Tripathi32 <[email protected]>
Co-authored-by: t-hamano <[email protected]>
Co-authored-by: tyxla <[email protected]>
Co-authored-by: Mamaduka <[email protected]>

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[Mayank-Tripathi32]

@t-hamano @youknowriad @mikachan Can you please have a look and review whenever possible 😊 Thank you.

[t-hamano]

Thanks for the PR!

I have one concern with the approach taken in this PR. Could you please confirm this comment?