Great Scott! Time sure flies fast! I remember watching 'Back to the Future II' many years ago and thinking 2015 is a long ways away... but we're here now and it's finally October 21, 2015 aka 'Back to the Future' day, when Marty McFly travels to the future.

With that in mind, let's take a look at some of the #1 hits on this day (October 21) over the past 17 years... Back to the Past!

Many of you may know this song thanks to 'Reply 1997'.

The legendary group back in their younger days... yo.

This was #1 just a month before her infamous sex scandal. Back when Baek Ji Young was on top of the K-pop world.

The South Korean mogul was quite the talented performer, evidenced in this performance of "I Have A Girlfriend" which was #1 on the charts back in October of 2001.

A popular artist back in the day. It's hard for artists now to sell 500,000 copies of their album but he would do it routinely more than a decade ago.

Hwanhee and Brian, what a combination that brought us numerous beautiful songs over the years.

The United States had Justin Timberlake and South Korea had Se7en. He was quite the performer back in the day.

If you don't know MC Mong, you should know. He's a talented artist with numerous #1 hits spanning more than a decade.

Nostalgia... it's kind of sad that the Super Junior members are getting older and heading to the military one by one.

If you think F.T. Island, you most likely think of this song.

Getting into the golden age of K-pop. A wonderful song that is timeless and was #1 on the charts back in October of 2008.

They took the K-pop world by storm in the fall of 2009 with this addictive song and dance.

Probably my favorite song from them, can you believe it's already been 5 years?

Haven't had a release in a while but hopefully they can return soon. Blackjacks sure miss them.

When 'Reply 1997' was all everybody was talking about combined with the exploding popularity of Seo In Guk and Eunji.

The era when many K-pop fans started to take notice of EXO.

It's already been a year since WINNER released "Empty." Time sure flies fast, hope you enjoyed this look at some of the past #1 hits on this Back to the Future day (October 21).