It's a part of a sentence. It's not a complete sentence because it lacks a main verb. As you started the sentence with a capital "T", I think that you think it's a complete sentence. Can you add a main verb?

I finished the college. . So i used ago

The most beautiful day is the day when I have graduated after 4 years. you have studied for 4 years ago. you have studied since 2011. you wait graduating from your college or school.