Internally, WordPress uses a special `post_date_gmt` value of `0000-00-00 00:00:00` to indicate that a draft's date is "floating" and should be updated whenever the post is saved. This makes it much more difficult for API clients to know the correct date of a draft post.
This commit provides a best guess at a `date_gmt` value for draft posts in this situation using the `date` field and the site's current timezone offset.
Props joehoyle, jnylen0.
Merges [40108] to the 4.7 branch.
Fixes#38883.
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