Let's start with #1183250: Add a theme_datetime() function to consistently theme dates and datetimes once the basic functionality is committed.
See #1183250: Add a theme_datetime() function to consistently theme dates and datetimes from comment 191 on for a stub patch and an explanation of this issue.
Basically, theme_datetime does not yet account for render arrays; it will only accept plain text or HTML strings until it is updated to incorporate the patterns of the render system.
Comments
Comment #0.0
jessebeach CreditAttribution: jessebeach commentedUpdated issue summary.
Comment #1
Anonymous (not verified) CreditAttribution: Anonymous commentedComment #1.0
Anonymous (not verified) CreditAttribution: Anonymous commentedUpdated to reflect problem.
Comment #1.1
jessebeach CreditAttribution: jessebeach commentedUpdated issue summary.
Comment #2
mgiffordI'm assuming we can close this issue as https://api.drupal.org/api/drupal/8/search/theme_datetime no longer exists in D8.