The date format in Smartermail webmail depends upon your browser's locale / language settings.

For Internet Explorer, change the Location settings in your computer:

Windows XP: Control Panel > Regional and Language Options > under Regional Options tab > Location

Windows Vista: Control Panel > Clock, Language, and Region > Regional and Language Options > Location

Windows 7 / 8: Control Panel > Clock, Language, and Region > Region and Language > Location

For Google Chrome (version 31+):

Settings > Show Advanced Settings > under Languages, click Language and input settings button

For Mozilla Firefox (version 24+):

Tools > Options > Content tab > under Languages, click Choose button