in the mate desktop environment that comes with linux mint, the clock is an applet built into the panel program.
to change the date and time format, press alt+f2 or open a terminal window. type "mateconf-editor" and press enter. this opens the configuration editor for mate. if you don't have this installed, type "apt-get install mate-conf-editor" in a terminal.
navigate to /apps/panel/applets/applet_clock and click on "format". read the long description and possibly set the value to "custom". if you use "custom", also change the "custom_format" field, which accepts the same format strings that strftime() uses.
i set custom_format to "%F %H:%M" which displays the time in the format 2012-10-13 15:17