For all those who are requesting ISO8601 dates -- according to the GitHub repo [0] these are already supported:
> Dates: A date can be expressed in a few forms. Human readable dates are supported, like 1665, 03/2222, or 09/11/2001, as well as IO8601 dates, like 2031-11-19T01:35:10Z.
> Dates: A date can be expressed in a few forms. Human readable dates are supported, like 1665, 03/2222, or 09/11/2001, as well as IO8601 dates, like 2031-11-19T01:35:10Z.
[0]: https://github.com/kochrt/markwhen