Learning about the Y2K38 problem will lead you to a way to decode the flag.
1721387471
The flag is in the format leveleffect{dd MMMM yyyy HH:mm:ss}
I searched up the issues around Y2k and found that it had to do with time and with the epoch in the title i assumed that i needed to find a converter.
I found the following site and got the output:
https://www.epochconverter.com/
Friday, July 19, 2024 11:11:11 AM
flag= leveleffect{Fri 19 Jul 2024 11:11:11}