[tantek]I believe I have seen two approaches, either integer seconds since epoch start (or finer grained if you post semi-automated or passive posts that could collide on a 1s boundary), or ISO 8601 dates (and times).
LoqiISO8601 is an international standard for dates and times roughly in the form YYYY-MM-DD HH:MM:SS, or with timezone offset, YYYY-MM-DD HH:MM:SS +0000 https://indieweb.org/ISO_Date
Loqirelative dates are a way of converting absolute date-times like year, month, day, and time into a more human friendly expression of time elapsed like "10 minutes ago" or "4 days ago" https://indieweb.org/relative_time
[social]2I showed up in Frankfurt about a month early for my return flight (their system had a calendaring problem when I booked the flight). I was returning on the 11th, but of which month. They put me on a flight the next day and put me up for the night and had a car service available.
[lazcorp]Decluttered my blog post template a bit more by adding another non-JS popup (using the global popover attribute) to tidy away details of the "Subscribe by RSS, Email, or ActivityPub"