Don't know if this is a bug or an intended thing.
I noticed today after 12 o'clock noon today, my new email from today was showing as 1600 instead of showing as 4:00pm.
I made the changes to the config/main.inc.php file and the dates / times for everything after 'today' was working as I inteded it to work. I ended up fixing this by going into the program/include/main.inc and changing the line below.
return sprintf('%s %s', rcube_label('today'), date('g:ia', $timestamp));
If you are seeing the same thing, this seems to be a quick and easy fix.