TimelyBills takes security of users data at-most priority and Private Mode is even a next step in that direction. As soon as you start using TimelyBills app in Private Mode, it stop synching with any of the internal or external system over internet. It guarantees to keep all of your financial data with in your device only (on which you are running TimelyBills app). 

Private Mode comes with 15 days trial and after that one has to purchase Pro Version to keep using private mode.
If not desired, one can simply login using any email and can come out of private mode. No data will be lost during this switch from private mode to non private mode.