Hello Gregg,
Step 1: Go to Customization > Tables/Fields > Activity > Edit Fields
Make a new field. Say EmailStartDate that is a DateTime. Click Add Field > Finished Editing Fields.
Step 2: Modify the invoice screen to save a value into your custom field with the proper formatting.
Assuming you have done a straight copy of the Activity screen, go to the screen designer for the page. Mine is CRM_NewActivity(2). Find the first Save button in the first section. Click Edit and go to the action list. Add the Date function (see Aushido's post) and an Update Field in Table action. (You can do an Enter Value Into Field On Screen action. You would need to put EmailStartDate on the screen and hide it. Also, the action would have to go before the Save All Sections action in the Save button.). Move both actions before the Save All Sections action in the Save button. Click Publish.
Here is the Update Field in Table action. It gets the current record from the Activity table and adds the date with the formatting defined in the Action Result.
Here are the two actions together.
Step 3: Add the field to the email template using Activity.EmailStartDate.