activate/deactivate and quiesce/unquiesce commands shouldn't change the db_conn_time element of an active database.
Probably something or somebody started to deactivate your database every day for some reason. Try to find some evidences of that in the db2diag.log...
Actually I did a mistake when I said that the 'quiesce db' command shouldn't change this element.
The 'quiesce db' command forces off all connection except the issuer's one.
If the database was not explicitly activated before (by an 'activate db' command) and you reset this connection which issued the 'quiesce db' afterwards, then the database gets deactivated since there are no connections in the database at this moment.