05/14/2012 03:58 pm ET

Sarah Cooper, American University Graduate, Receives Marriage Proposal At Commencement (VIDEO)

May 13 will always be a memorable date for Sarah Cooper.

On Sunday, the American University graduate walked across the stage to receive her diploma, but what she didn't expect was the diamond ring and marriage proposal that awaited her on the other end.

As Cooper prepared to exit the platform, Sam Miller, her boyfriend of four years, got down on one knee and popped the question in front of family, friends, professors and peers.

The very shocked graduate nodded a confident "Yes" and ran into her fiance's arms, causing the crowd to erupt in applause as the couple kissed and Miller slipped the engagement ring onto Cooper's finger.

According to WPRI, the college sweethearts promised their parents they'd walk across the stage before they walked down the aisle.

And Miller, who also graduated from AU, took that statement to heart as he prepared to make his big move.

"I knew that I wanted to spend the rest of my life with Sarah about three months into our relationship," Miller told NBC Washington in an email. "I chose to propose on stage at her graduation because I don’t want to wait another minute for her to be my fiancee."

A video of the proposal was posted to YouTube by American University later that day.

Watch the surprise proposal in the video above.