“that are capable of showing hidden files”… but if app create folder and file in (example) /mnt/sdcard/etc/andr/som23
is very difficult user know in that directory is file with date.
App must create folder and file in a strange place (path) so that the user does not suspect that the date is there.
In link of by @Andres_Cotes, you we can look Tercer método, app get DeviceId and load this number in a remote hosting with PHP and MySQL.
When user run app, app connect with remote MySQL and check DeviceID (necessary internet)
This is an example of Tercer método, is in Spanish:
http://kio4.com/appinventor/395_veces_funciona_app_MySQL_Exten.htm
In that example user can run app 3 times.
[first time, not exist number DeviceId —> create]
[others time suma +1]
…
That project need extension: TelefonoDatos for get DeviceId (IdDispositivo)
Extension TelefonoDatos (info. phone) and extension Localizacion
in http://kio4.com/appinventor/231_extension_infotelefono_cellid.htm