(ABC News)

By DAVID WRIGHT and BRANDON BAUR

What if you could remember every single day of your life and have it available for instant recall?

Everything from childhood birthday parties, Christmases, your first day of school, and your last. What were you doing that day? Who were you with? What day of the week was it? What did you have for lunch?

Actress Marilu Henner says she can remember it all. And you probably remember her as Elaine Nardo from the hit TV show "Taxi."

She can recall, off the top of her head, the exact day she got the part.

"It was June 4 of 1978. It was a Sunday and I found out at the 'Grease' premiere party," Henner said. "'Taxi' is so vivid to my mind. The very first rehearsal was July the 5 th of 1978. That was a Wednesday and our first show was shot the 14 th, a Friday."

The actress, who has also starred in "L.A. Story" (1991) and "Man on the Moon" (1999), is one of only 12 people in the world diagnosed with hyperthymesia, also known as Highly Superior Autobiographical Memory.

Most of us can remember major events of our lives - our wedding day, for instance, or where we were on 9/11 or when President John F. Kennedy was shot. But Henner, 60, can remember specific details from almost every day of her life.

She said her earliest memory is of being baptized.

"I just remember the water, and I remember the white," she said. "Whenever I go back into memory, I'm always in my body looking out."

Henner said her memory works like a scene selection menu on a DVD, with "little videos moving simultaneously."

"When somebody gives me a date or a year or something, I see all these little movie montages, basically on a time continuum, and I'm scrolling through them and flashing through them," she said.

She said it is a gift that helped her as an actress, but not in the way you might think.

Sure, she has an easy time remembering her lines. But, perhaps more importantly, she can call to mind moments of great emotion from her own life - to help her embody her characters.

"I can always remember where I first read a script or what I studied or what I liked about, things like that," Henner said. "But definitely being an actress, I learned how to embrace my memories and celebrate them and explore them without hesitation whatsoever."

In her new book, "Total Memory Makeover," in stores on April 24, Henner tries to help others unlock their memories.

Ironically, the actress' husband makes a living publishing the one thing in the world she doesn't need: calendars. Her son Nick Lieberman, 17, said he finds his mother's memory "comforting."

"Just this idea that everything that I've ever done is documented somewhere in her mind," he said.

Henner said her gift is not just a parlor trick. It's important stuff.

"It's that defense against meaninglessness," she said. "I'm not just occupying time. There's some significance to what I'm doing and how I'm living my life."

She said understanding your past is the best way to prepare for the future.