While a simple script could insert dates into a text field, a related table in a portal would work better for this.
Say your current table is called "Clients".
Add a table called EmailDates with this relationship:
Clients::__pk_ClientID = EmailDates::_fk_ClientID
If you enable "Allow creation of records via this relationship for EmailDates, you can place a portal to EmailDates on your Clients layout and format a date field in the portal with the drop down calendar. To log an email date, just enter this field and select the date.
A script could also be attached to a button to log today's date as the next EmailedDate record:
Set Variable [$ClientID ; Value: Clients::__pk_ClientID ]
Go To Layout [EmailDates]
Set Field [EmailDates::_fk_clientID ; $ClientID ]
Set Field [EmailDates::Date ; get ( CurrentDate ) ]
Go To Layout [original layout]
With such a script, you can set the portal to sort Emaildates by date in descending order to put the most recent email date at the top of the portal. You can also set up the EmailDates::Date field to auto enter the creation date and then leave out the second set field step in the above script.