When two people in the 'talking phase' meet up initially and do date type things without planning on it, only to later realise that their meeting could qualify as a date.
Post hoc date refers to a date that is officially recognized or labeled as such after the fact, rather than being a planned or intentional first date.
The couple laughed about their first meeting, realizing that it was actually a post hoc date, as they had already grown close through weeks of getting to know each other.