Type FIELD on the command line and you will get this box:
Select a field name and format, such as the date field in this case, and the final result will look something like:
When you use the FIELD command AutoCAD actually creates a field inside a multiline text (MTEXT) object. You can edit this by double-clicking on the object (or type DDEDIT), and then double-clicking again on the field itself. This will bring the Field dialog box back up where you can make modifications.