When an UE receives one of the following messages:
1. RRC Connection Reconfiguration with mobility control info.
2. MobilityFromEUTRACommand message including cell change order.
It start T304 timer. This timer is used to guard handover or cell change order.
If UE handed over successfully then stop timer else at the expiry of timer T304 start RRC Re-establishment Procedure.
Please correct me if I am wrong.