When using the preview button, Date Modified requires a time to save

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Describe the bug

When using the preview button and not setting a time for modified, you can see the preview and it has a time of 00:00:00 but when returning to the node edit for there is an error that a time is required. Adding a time is not required if the preview button isn't used and the node is saved first.

Steps To Reproduce

1. Open a DKAN admin site with the preview button re-enabled.
2. Create a new dataset
3. Set just the date for the Modified field and all other required fields.
4. Before saving, click the preview button
5. On the preview page, click the back to dataset creation button
6. You should now see the an error that a time is required
7. Try to save the dataset and the error will prevent this action
8. Set a time and successfully save

As a control, on the same site create a dataset and add a modified date with no time and save the dataset. It should save successfully and now you can preview it and come back and the error will not appear.

Expected behavior

A default time that is added during save is also added when using the preview button.