I would imagine you have the export the block that has the league card date, then hex edit that block, then reimport the block. No idea what block it is tho.
While I documented the offset of the timestamp (and other data) for the WR8, no idea how it works. (It's different from the epoch timestamp they previously used.)
Unrelated: Perhaps stop calling it "pre-ordered content or DLC". It's not.
Granted, some events are locked to pre-orders, that barely makes them cross the threshold into pre-ordered content.
I don't think anyone calls Events "DLC".