From: http://sysadmin.wikia.com/wiki/Ubuntu
gsettings set com.canonical.indicator.datetime custom-time-format "'%F %H:%M'"
gsettings set com.canonical.indicator.datetime time-format "'custom'"
gsettings set com.canonical.indicator.datetime custom-time-format "'%F %H:%M'"
gsettings set com.canonical.indicator.datetime time-format "'custom'"