how to upload PO's in earlier date to the system

Summary

i want to upload 50 PO's having the date 2018 from the legacy sytsem to the new system having the same date

Content

we are implementing Oracle cloud , and we have around 50 open PO's in 2018 that we want to upload through a batch file or data loader, the implementer says 

they cannot upload them to be in 2018 , they should take the date of 2019 as same the creation date !!

but we dont want them to have 2018 date because these are 2018 PO's , we want them 2018 

same exactly wen you want to hire an employee in earlier date , lets say we are in October 2019 and we want to hire him in September 2019 so we go in the date for earlier date and hire them , so why cant ii do the same with PO's