Word’s Date and Time Feature

From the Insert ribbon, click on Date & Time found in the Text grouping.  A pop-up window will appear allowing you to select the desired format.

Here you can Set As Default once you have highlighted the format you need.  Also, make note of the checkbox to Update automatically.  When checked it will change the date to the current date any time you open the document on a different date.  If you uncheck this option, it will place the current date in the document when you add it, but it will not change that date once it is in the document.

You can then add the command to your Quick Access Toolbar by right-clicking on it in the ribbon and selecting Add to Quick Access Toolbar or remember to use Alt + Shift + D to insert the default date format you have chosen.