When filing for state, reporting purchases made out of state that don't invoice sales tax, should i only report for the calendar year or up until tax returns are due?

I made online purchases out-of-state from NY to PA, which has a 6% sales tax, in the 2018 calendar year. I'm just wondering if I should interpret the tax form to report those expenses as exclusively for the 2017 calendar year or should I include the period of time up until filing for 2017?