Generally in fail2ban all the times are in unix time, so all come in/out as integers. Sqlite3 also doesn't have a standard datetime type, but it appears that python will convert datetimes to ISO string format. 
I think it would be good to make use of datetime more throughout fail2ban. If this is something we are thinking of doing, it might be indeed worth making this a datetime type in the db now, such to avoid a schema change in the future?