Segment Insights do not backdate as product tagging can change overtime.
When you create a new custom segment, all the customers who visit your online store and who are eligible to belong to the segment will get all events related to their visit aggregated to the Insights of the segment.
However, if you have been using Nosto for some time already, Nosto is able to use all data tracked since the implementation of the solution.
For example, a customer who has ordered one time in January 2017 revisiting your website in August 2018 is instantly recognized on landing as a 1st-time Buyer. This is only valid is Nosto was installed in January 2017 and has tracked the initial order.