The 41-year-old actor testified that when he eventually arrived at her home, he "knocked on the door and there was no answer".
Kutcher's date Ellerin had plans to go with the actor to a post-Grammy Awards party on February 21, 2001, the night she was murdered at her Hollywood Hills apartment.
The Los Angeles trial continues.
Gargiulo - who has a 1997 felony conviction for burglary in Cook County, Illinois - lived one block away from the high school graduate at the time of the slaying and was good friends with one of her two younger brothers, according to an arrest warrant filed in Cook County. He remembers the exact time because of the state of their phones. He said he called Ellerin nearly an hour later and left a voicemail saying he was still running late. He arrived between 10:30 and 10:45 p.m. and noticed a security gate was open.
The prosecutors say that Kutcher's testimony is meant to establish the "chronology of this case", and that it will show "there was a... very narrow window of opportunity for someone to get into the house and murder Ashley, and whoever that person was must have been watching".
He figured she left with her friend because he was late.
"I was trying to take her on a date and didn't want to seem eager", Kutcher testified.
"I thought it was odd that lights were on and she wasn't there", he testified.
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When Kutcher stopped by her home, he peered through the window and saw red wine stains on the floor.
After Kutcher found out what really happened to Ellerin he called the police since his fingerprints were on her door and was admittedly "freaking out".
Along with Ellerin's murder, Gargiulo is charged with the December 1, 2005, killing of Maria Bruno in an El Monte apartment and the attempted murder of Michelle Murphy, who survived being stabbed eight times in her Santa Monica apartment in April 2008.
Deputy District Attorney Dan Akemon explained Gargiulo's motive at the start of the trial on May 2, calling Gargiulo's plotting to murder women with a knife in and around their homes "his hobby", according to reporting by CNN affiliate KABC.
According to People, Gargiulo lived in Ellerin's neighborhood and offered to help her fix a tire outside her home in 2000.
In 2008, Gargiulo accidentally cut himself when he allegedly attacked Michelle Murphy, KABC reported, allowing investigators to link his DNA to the killings.
"All of the victims were young, outgoing women", Akemon said.
Michael allegedly had killed her that night by stabbing her 47 times.