This is usually done at the Form-level in the Form_BeforeUpdate event, which fires anytime a new or edited Record is saved. If you used the Default Value, at the Table level, it would only apply to a New Record, and you'd still have to do it at the Form-level, in order to change it for an edited Record.
BTW, if you use Now(), you'll get both the Date and the Time element in a single field.
Linq ;0)>
The problem with making anything foolproof...is that fools are so darn ingenious!
All posts/responses based on Access 2003/2007