Hi there thanks for posting in the Lucid community! To clarify are you referring to the date options under the Insert menu? If so unfortunately the options are limited to the month:day:year and day:month:year options shown in the screenshot below and customizing the format isn't currently supported in Lucidchart. 

However I definitely understand how more options or the ability to customize them would be useful. I've logged your feedback for review by our development team which considers requests like this when prioritizing improvements to Lucidchart. If you're willing we would love to hear additional details about your use case and how this customizability would be beneficial to you either in this thread or via our feature request form. Thank you again for your feedback and apologies for any inconvenience. Please let me know if you have additional questions!