Assange was to be released from Belmarsh prison on September 22. He then served the sentence for violating the bail release.
The judge states that Assange has departed earlier, the BBC reports.
The UK government approved last summer's US request to have Assange extradited. There he is suspected of, among other things, data breaches by breaking a password to one of the US Department of Defense's computer systems.
The judicial process in the British court to decide whether Assange should be extradited to the United States is expected to take place in February.
In Sweden, prosecutor Eva-Marie Persson has held several testimonies regarding the case of rape when Assange visited Sweden in 2010. Assange denies the charges and has refused to travel to Sweden to be questioned.