Writing the current date by a keyboard shortcut

You could create a Automator Service/Quick Action, where the workflow looks like this:

Once this is saved (e.g. ISO8601_Date), visit System Preferences : Keyboard : Shortcuts : Services. It will likely be found in the Services Text section under the name that you gave it. Services without an assigned keyboard shortcut will show a none in their right column. Click the word none. This will change to Add Shortcut. When you click it again, you can enter your unique shortcut. I gave mine control-D:

If you assign a shortcut already used by the operating system, your choice will be ignored. Same drill for the application that you use it in. The first time that you want to use the Service, look in your application's Service menu for it, and invoke it from there. Afterwards, the keystroke should work.

Here is the output of this Quick Action (shown in white text) in Mojave 10.14.6 as shown in the Terminal:

For Word®...

If you are insistent on using a keyboard shortcut, you will need to use Shift+Alt+D to insert the date, press the left arrow key until you are in the middle of the inserted date, and then press Shift+Ctrl+F9 to unlink the field.

https://word.tips.net/T003418_Different_Ways_of_Inserting_Dates.html

https://support.office.com/en-us/article/insert-today-s-date-in-a-word-document-fe7c08fe-7192-44a5-ac0a-88ccd51532ab

Too early, and the coffee hadn't worked its magic yet. Do not assign a control-D to this shortcut, or it will impact the keyboard shortcut necessary to end a Terminal session.

You are welcome.

Some applications which already use the chosen keyboard shortcut may block your Service, and you may have to run it from the application's Service menu instead. Even then, it may not work depending on how that third-party application was written.