When does dating officially start in Korea?

On the day one person asks to date and the other agrees. That moment is called gobaek, a confession, and that day is day one.
It does not work by drifting into a relationship over a few dates. You can meet every week and text every day, and until that sentence is said, you are not officially together.
Korean has a separate word for the stage in between. It is called some, pronounced like the English word. Both people are interested and neither has defined it, and it is not expected to last long.
Because the start date is exact, it can be counted. This is where the habit of marking 100 days, 200 days and 1,000 days comes from.
K-pop lyrics and K-dramas are built on this structure. The confession scene works as a turning point because the relationship genuinely changes on that day.