Along with your 3DS, just change your Computer time and date to the date the event was active, BEFORE opening PKHeX or downloading or saving the wonder card event. I left an example below, sorry if anything is misspelled or anything, I was in a rush.
Example to this Wonder Card Date fix Problem:
Lets say you want the (2018) Get a Legendary Pokemon! "Shadowy Yveltal!" Wonder Card, which was only available/redeemable from May 4th 2018 to August 28th, 2018.
Step 1. Make sure everything on your computer is closed, internet, photoshop, games, PKHeX, etc.
Step 2. On your computer, go to Settings>Date & Time.
Step 3. In the Date & Time settings:
"Set time automatically" - Make sure this is turned OFF.
"Set time zone automatically" - Make sure this is turned OFF.
"Change the date and time" - Change the Date to the date the event was active, so from May 4th 2018 to August 28th, 2018. I will set my date to May 4th 2018. (Note you can only change this when everything else is turned OFF)
"Adjust for daylight savings time automatically" - Make sure this is turned OFF.
Step 4. Close out of settings and open PKHeX.
Step 5. In PKHex go to Tools>Data>Mystery Gift Database> and find the Wonder Card your looking for. Mine is the "Shadowy Yveltal!", Right Click on the event and "Save Gift" to Desktop, or whatever folder you choose.
Step 6. Load up your games Sav. file into PKHeX, then open the "Mystery Gift" box and drag and drop the event you just saved to your desktop into your events, hit save, then save your Sav. File.
Step 7. Take your SD/Micro SD card out of your computer, and put your SD/Micro SD back into your 3DS.
Step 8. Turn your 3DS back on, and go to your settings and change your 3DS time and date, to the time and day you set your computers too. So, I would set my 3DS to May 4th 2018.
Step 9. Close out of settings, open checkpoint or whatever you use to restore your new Sav files to your game.
Step 10. Start your Game and go collect your Event Pokemon, after that save your game and check your wonder card.
Step 11. Restore your Computers time and date settings and your 3DS time and date back to normal.
Hope I helped.