Date and Time format in Lync Web App 2013
In Lync Web App 2013 the format used when displaying date and time is taken from the language you have chosen when starting up Lync Web App. It is not using your browser language settings or your desktop date and time format settings.
In the screen shot below English has been chosen. If you click on the language you are able to change to another language to match your date and time format preferences.