In Shopify, you can specify a relative date and time for the last purchase date, so you can create a segment of inactive customers who haven't made an order in the last n days.

Shopify's customer segments can be extracted using a variety of conditions, but this example shows how to extract inactive customers using one of these, "Last Purchase Date." The condition shown in the drop-down menu, "<=", means "before date," but you can also specify "relative date (month)," so you can extract accounts that haven't made a purchase in the last n days from the current time.