In SAP, (46 or ECC),
to get PR/PO Approval/release date and time, you can use this function module:
Document_Category value: you can pass B for PR and A for Purchase Order,
and put in the PR/PO Number in Document_Number.
to check the release time and date, you can loop on XCDHDR internal tables, as the function module result.
this function module works much2 faster than we select directly to the CDHDR table.