Seems that in a little source inspection, the "ul" tag issue is related with unclosed tags.

For the datetime format, is the space between the hours and the timezone, the meta tag with the same info has a valid format too which could be used for time tag.

Edited: I report the datetime format issue because it is maintained even if I change the format in the source file, seems that the error is between the steps. I think it does a kind of conversion and adds extra space between the time and the timezone if not exists (yes, I tried using the same format without that space in the source file of the post and didn't work).

A partial workaround is adding https://github.com/apsislabs/jekyll-tidy to the Gemfile.