The calendar representation may be off by one day in some cases. It just happens that the module uses the same article date for all dates, but the calendar representation needs some PHP in order to separate date elements into chunks.
You may end up with a difference of 1 day whenever the php.ini on your server and your Joomla! configuration are not in the same time zone (depending on how far away they are apart and your time of the day, you will end up with 1 day difference).

Hello,
no, this has not been resolved yet, mostly because it is a configuration issue and if servers are setup correctly the issue does not arise. Another major hurtle is that I had to make the code compatible with older versions of PHP so it works for everyone. More reliable PHP functions exist but starting version 5.3. By making a specific version of the module for Joomla! 3 (which requires PHP 5.3), I will be able to 'force' those functions into the code.

It is rather strange. Either the modules would all be off by one day or not, depending on the server configuration.
What type of dates are you showing for each module (modified...)? Are the type of dates the same for each module?

Note that items may seem off by one day, but it may be because the server is off by a few hours. An article saved at 11pm may show up for the next day because the server is off by 2 hours (as an example).