Reeves held the weapon behind his back and unfolded it.
He concealed it by his side as he stepped towards the victim.
"He held the knife to his neck, pausing briefly, and slashed across his throat in a quick and deliberate manner," the judge said.
A man unknown to the defendant then punched the victim in the head.
Both were ejected from the bar and the victim was left with a small cut on his chin which did not require medical attention.
Despite the relatively minor injury, Judge Phillips said it could have been much worse.
"This was at the highest degree of recklessness," he said.
Reeves and the victim sat down for a restorative-justice conference during which an apology from the defendant was accepted.
He admitted the man had done nothing to deserve his violent outburst, the court heard.
The victim said he had become paranoid since the incident and had given up his job as a result.
Judge Phillips gave Reeves credit for attending the conference but said his subsequent comments to a Probation officer offset that.
In a pre-sentence interview, the defendant called the man "aggravating and annoying", which the judge regarded as an attempt to deflect the blame.
Reeves said he was carrying the knife because he had been attacked a week earlier, the court heard.
He was convicted of injuring with reckless disregard.
Defence counsel John Westgate said a prison term would be difficult for his client because he suffered from liver disease.
Reeves was assessed as a medium risk of reoffending and high risk of harm to others.
