An Arlington Circuit Court judge has overturned the conviction of a man for rape, robbery and malicious wounding because his attorney was confused about the time the crimes occurred.

In a decision released yesterday, Judge William L. Winston ruled that D'Alesandro Barber, 27, did not have "effective assistance of counsel" when he was found guilty last December of raping and slashing a woman after breaking into her first-floor apartment in the middle of the night. Winston ordered a new trial. Barber's attorney, Edward C. Farstad, had acknowledged in court that he went to trial believing the attack occurred on Sunday night, July 11, 1982, when the incident actually took place about 2:30 a.m. that day.

Farstad initially told jurors the defendant was home asleep in bed that night and did not have a "red hot alibi." Barber later testified that he was at a rock concert with a cousin the Saturday night before and Sunday morning of the incident. But a jury found him guilty after the victim identified him. The jury recommended a 45-year prison sentence.

Farstad, a member of the Arlington Bar Association since 1974, has substantial experience in representing clients in criminal cases, according to court papers. He said after yesterday's ruling that the judge's decision was proper. "I had information that it happened on Sunday night . That information was incorrect," he said. Farstad said much of his confusion was due to his "horrendous schedule" at the time.

Assistant Commonwealth's Attorney Helen Fahey, the prosecutor in the original trial, had no comment yesterday. She said a new trial would be scheduled to begin within the next two months.

Richard C. Shadyac, Barber's new attorney, had asked that the guilty verdict be set aside because of ineffective counsel. He also contended that Winston erred when he refused to declare a mistrial and disallowed the testimony of Barber's corroborating witness, though the judge did not rule on those issues yesteday.

Attorneys in Shadyac's office said yesterday that they will file motions in court this week to get Barber, in jail since his Dec. 8 conviction, released.