Quick tips for you on how to configure the panel DateTime display in Linux Mint Cinnamon.
The DateTime Widget contains a helpful “Show information on date format syntax” button which is often overlooked. Click it!
You'll see a number of options on the resulting web page. Simply choose one of the pre-set formats and paste its pattern into your widget's Date format field.
Also observe the 3 tabs on the “Show information” web page. Between the 3 resources you should be able to configure your Mint Panel Date Time in about 12 seconds.