THERE'S no time like the present to get "Back to the Future," a temporal fantasy that looks as fondly forward as it does nostalgically to the past. Starring Michael J. Fox as a time-tripping teen named Marty McFly, it spans 30 years but seems like minutes. You see, time McFlys when you're having fun, and this is undiluted comedy, equal parts laughter and love.

Fox, also the star of TV's "Family Ties," shares this comic showcase with Christopher Lloyd, who's best known as the Reverend Jim of "Taxi." Lloyd is in mighty form as Doctor Emmet Brown, the nutty inventor who sends McFly back to the '50s in a gull- winged, plutonium-powered DeLorean. Equipped with vanity plates that read "OUTTATIME" and a flux capacitor, it speeds back to the past like, well, a sports car on cocaine.

Naturally, there are the usual problems that face anyone traveling in time, like how to open a Pepsi without a screw-top cap. And if you change the past, do you erase yourself or what?

Upon arriving in 1955, however, McFly discovers that time is relatives. He meets his father (Crispin Glover) at a soda shop; his grandfather runs him down with his car; and his mother (Lea Thompson) nurses him back to health, believing him to be Calvin Klein -- the name on his underwear.

Mom, destined to become an alcoholic in the '80s, is all peaches and cream and prom dreams now. Dad, destined to failure, is sweetly clumsy as the original nerd. But the family's future alters drastically when mom gets a crush on her own son and won't date dad. The possibilities are incestuous, but tamely and tastefully resolved.

Robert Zemeckis ("Romancing the Stone") directs the rollicking, interlocking script he wrote with college classmate Bob Gale. And executive producer Steven Spielberg also makes his marks -- for instance, with a theater marquee to set the scene. Ronald Reagan is starring in "Cattle Queen of Montana" in the quaint town of Hill Valley, which had been suburban sprawl only moments before.

Here McFly finds he's fresh out of fuel with no way back to the '80s, aspartame and heavy metal. His only hope is to find Doc Brown in 1955 and figure some way to generate the billions and billions of jigawatts needed to go home. The two of them plot McFly's return while the young man attempts to reunite his parents by prom night or else never be born.

It's a sensational variation on a timeless theme, expertly directed, written and performed. It makes you remember your past and forget your troubles all at once. But wait. It's 1985. Do you know when your children are?

BACK TO THE FUTURE (PG) -- At area theaters.