Donaldson had previously admitted a charge of making threats to kill.
The court heard Donaldson had drunk 1.5 litres of vodka before the encounter on the afternoon of 22 March.
CCTV played to the court showed Mr Miliband walking in the street with Donaldson crossing the road, gesticulating with his hands.
The threats were followed by him saying “I’m going to do it now” before reaching into his pocket, said prosecuting barrister Ashleigh Metcalfe.
Mr Miliband, who was visiting a local charity, felt “instantly vulnerable” with him running towards the building he was due to visit.
Once inside, the doors were locked with the defendant seen through the glass outside.
It was the first time Mr Miliband had been threatened in such a way, the court heard.
