Most stores do not include a Ship By date as part of the order information sent to ShipStation (with the notable exception of Amazon).
However, to ensure your staff ships an order on time, you can add a Ship By Date to your orders in ShipStation. This information will then display in the Order Details and in the Ship By Date column, which you can use to view and sort orders by in the Orders grid.
To add a Ship By Date:
Select the order(s) in the Orders tab you'd like to set a Ship By Date for.
Click Bulk Update and choose Ship By Date.
Select the desired date and click Update Selection.
You can automate this process using Automation Rules so the Ship By Date is set as orders import. Review the Automation Rules article for details.