Thank you for the request, Petr! As far as I understand, SmarterMail should mimic the date and time format as it is displayed by your browser. However, we've aware of some areas that are incorrectly formatted and some other odd behavior. This has been reported to the development team and will be reviewed.
It's likely SmarterMail will continue to rely on the browser locale for the formatting of the date and time displays, as I imagine this is the simplest implementation for that functionality. However, are you interested in specifying the exact date and time format, rather than mimicking the browser locale? Choosing from 24-hour vs 12-hour and picking a specific date format: 2.brezna 2014 vs March 2, 2014, 2.3.2014 or 3-2-2014?
Look forward to your thoughts on this functionality.