Update the published datetime regex

[wilwade]

Problem

Published regex did not support fractional seconds or other timezone offset options #179076218

Solution

Updated the regex

Double Checks:

• [x] Did you update the changelog?
• [x] Any new modules need to be exported?
• [x] Are new methods in the right module?
• [x] Are new methods/modules in core or not (i.e. porcelain or plumbing)?
• [x] Do you have good documentation on exported methods?