EDIT: Not reproducible on meta - investigating further.
Inserting a date and then clicking on the resultant widget causes the panel that is displayed to appear ‘beneath’ any following posts.
in case it is not clear from the screen-grab, the orange rectangle is the panel containing the list of timezones for that local time. It clearly goes ‘beneath’ the post below.
I’m able to reproduce this on our live site under the latest v2.3.0.beta1 +8.
Simply insert a date into a post, then post a reply to the topic so there is at least one post below the post with the date widget on it.
Then click on the date widget to get a list of the equivalent times.
I guess the list of time zones has to be long enough so that the panel that is produced is deeper than the height of the post it is in.
I’ve disabled a bunch of plugins and it still happens, so will try to reproduce it on meta in the posts that follow.
Oddly enough, the same topic when viewed on mobile actually works.