Another Max Gaetanno Mystery

It was eight o’clock in the morning when detective Max Gaetanno was called in to the case of history Professor Peter Hilton’s murder. The professor’s secretary, Mrs. Holden, was shocked this morning at the sight of him, dead on his desk in a hideous way, as if the last moments before his demise were spent with helpless struggle.

Detective Max reached the professor’s office a quarter of an hour later to find that the city police has already arrived along with three individuals who were closest to the professor during his life: Mrs. Sarah Hilton, Peter’s wife; Mr. Jason Jujube, Peter’s best friend; and Mr. Robert Hunnington, Peter’s lawyer. The office was gloomy; lights were broken down, walls were scratched, and pieces of furniture were scattered on the ground. Lying about the room was an infinite number of torn papers and books of history. “An act of vandalism,” the detective described it.

As the detective approached the body, he began his investigation.

-“Mrs. Holden, when did you left the place yesterday?”
-“I leave the office at four,” answered Mrs. Holden instantly, dying to show her innocence.
-“Had the professor any appointments afterwards?”
-“No,” she answered hesitantly, “but the professor said that three people would come to visit him in his office.”
-“And that means we have all the suspects here?”
-“Yes! The professor hasn’t many of visitors.”
-“Have they been investigated?”
-“Yes. The three gave times of their visits, saying he was okay when they have arrived. None of them met the other in the office. Expectedly, the police couldn’t tell who was honest or not.”

The professor’s back had several stab wounds. His left hand looked bloody and was dangling to the floor.

-“What’s with his left hand?”
-“The police said that its bones were crushed to dust. They assumed the murderer stepped on his hand with hardheartedness.”

The detective looked suspiciously at the desk. The victim’s head was lying silently, surrounded by an enormous number of papers and books; none of which caught the detective’s attention except the spot towards which the victim’s right hand -ringed on its wrist with a priceless watch- was awkwardly stretched.

-“Has the calendar always been on the desk at this hard to reach distance?”
-“No. It seems that it has fallen accidentally off the wall during the presumed fight.”

The detective barely heard what she has said, as if he had already known the answer. “Perhaps his love for history made him strive to write his last -but everlasting- message on the face of history: the calendar,” thought the detective. He was now looking at the pen lying beneath the victim’s hand: both placed on the calendar on which the following was written:

7B91011
-“Does that mean anything to you?”
-“No.”

A spark of enlightenment struck through the detective’s head.

-“Well, it does to me. I know the murderer!”

Who was the murderer? Why?