Here is a sample of data from a recent data feed file - diff is simply the difference between the two columns. In many cases, there is no difference, but a large percentage of fields has a first_hit_time_gmt that differs significantly from the visit_start_time_gmt. My question is... why?
visit_start_time_gmt : Timestamp (in Unix time) of the first hit of the visit.
first_hit_time_gmt : Timestamp of the very first hit of the visitor in Unix time.
So, visit_start_time_gmt refers to the visit and first_hit_time_gmt refers to the visitor i.e. For New Visitor visit_start_time_gmt will be equal to first_hit_time_gmt but for returning visitors it is not.