Test date in YAML front matter

[bernhold]

Check that the date provided is properly formatted.
Check that it includes both a day and a time.
Check that the time includes a time zone.
Sanity check that the date is no earlier than 2016 (when the webinar series started)???
Check that the time zone corresponds to either EST or EDT.
Check that the time zone (EST or EDT) is consistent with the dates for daylight saving time for that year.
Warn if the date is not a Wednesday (sometimes we have to adjust).
Warn if the time is not 1pm ET (sometimes we have to adjust).