Fairly self explanatory. This is USUM.
the newer games run off of a online server for events, thats why whenever you enter a code for an event, you connect to the internet. that server keeps the time and date. it does not go off the system time, but the servers time/date.
Unfortunately changing your 3DS time won't get access to older events, so if you want events from gen 6 (like XY, ORAS) you can't, isn't there a way to build a real life time machine to warp to the future
