Sqlite unique id in cloud solution

I have an application that work on mobile devices in offline mode, then the data sent to server later on.

Currently i'm using Timestamp as Table id, to generate unique id between mobiles devices. every thing was working fine until i got 2 different records from 2 different devices with same ID (timestamp).

*******the mobile app is and android application

the time stamp is generate using Calendar.getInstance().getTimeInMillis()

how can i get a unique id by mobile device knowing that i have i huge number of class/entities that use the same logic