As it turns out, the same solution in the previous post should apply.
The Form_BeforeUpdate event occurs whenever a modified record is about to be written, which seems to be just about what you need in a DateModified field.
So, all you have to do is
1) Bring up your data entry form in Design View.
2) Find the Form BeforeUpdate event in the events tab
3) Select EventProcedure
4) Click the build button (three dots)
5) add the code above (me.ModifiedDate = now())
6) compile and save