I succeeded to add a date to the subject by modifying the sendmail.py file, an example:
yesterday = datetime.datetime.now() - datetime.timedelta(days = 1) message['Subject'] = subject + " " + yesterday.strftime("%d.%m.%Y")
It is located in the /opt/splunk/etc/app/search/bin/ directory.
It is a python script and it can be modified with any text editor. Keep in mind that it will be probably overwritten on the next Splunk update.