Flirting = Dating ?

Mary • My beliefs and opinions are constantly evolving
There's a senior in my anatomy class and I noticed a ring on her marriage finger and asked about it. She told me she was engaged and planned to marry after they both graduated, and when I asked how long they've been together, she said "basically almost 5 years"
So I was confused and just went "basically??" And she explained to me how they flirted for 3 years (and did nothing more) but didn't go out until last year, and their 2nd year anniversary of dating is coming up. 
So does flirting count as dating now or something?? I was just confused and stuff