Since when do you start counting the length or period of time of a relationship?

We first met almost a year ago but only after a few month we got connected and start dating. There was something in the air if you can describe it right. It took us a while longer to date. Today we officially recognized as a couple. Probably I am going to propose soon.

The problem is I can not define how long we’re a couple. One year? Six months? How many months? I know there are certain dates couples are celebrating but it is not clear. So how and when do you start to count? First date is the beginning of a relationship or even first kiss or first sex?

Yesterday a good friend asked me how long we’re together and this bothered me, that I can not to tell him exactly how long.