Whats the difference between Form entry Completed time and Received time?

On the app side, the device's current date/time is used as the "Completed" date/time of the entry.

This time is only as good as the device.

So if the device time is not accurate, then this will reflect on the Completed value as such.

However once the entry data is uploaded to the platform, our system timestamps the entry with what we call a "Received" time.

This is an accurate server time that cannot be changed or altered.

So you have effectively two times concerning each Form entry - a device-specific Completed time and a platform-specific Received time.

The Received time may differ from the entry Complete time if the user was offline when completing the entry.  

If the user is offline for a substantial amount of time, then this time lag between when the entry is completed on device and when the entry is received by the platform can be substantial.