Pre-publish panel calendar and timepicker are misaligned at certain breakpoints

[sarahmonster]

Issue Overview

In the pre-publish panel, the calendar is left-aligned, whilst the timepicker below it is centre-aligned. This works at the majority of viewports where the sidebar is a controlled width overall, but falls apart at those viewports where the sidebar ends up being the full width of the screen:

Steps to Reproduce

1. Set your browser width between 320 and 778 pixels. (Approximately.)
2. Start a new post and add a title.
3. Press publish.
4. Click on the "Publish" panel to open the datepicker.
5. See misalignment!

This issue doesn't seem to happen elsewhere—the only other usage of the calendar component I could find was in the document settings sidebar, where the calendar appears as a popup instead:

Possible Solution

Basically, this would be resolved either by setting a max-width on the timepicker, or left-aligning the timepicker. The former is more elegant and in line with the calendar component as used elsewhere, but does rely on the calendar retaining a set width.