system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: In terms of hacking, what are zero days?
gpt
It's the number of days that the problem has been revealed outside of the hackers who found it.

For example, if Home Depot sold a door lock, but it had a problem where you could stick a magnet on it and it would unlock the door, then that would be a hack burglars could use to break into anyone's house who used that lock.

If Home Depot discovers this problem before the burglars do, they could publicly announce it and tell everyone who owns that lock to get it fixed. Then it's a race between home owners to fix their locks before burglars use the hack to break into their homes.

The more days that pass between the public announcement and a burglar trying to hack someone's lock, the more likely it is that the home owner has already fixed the lock.

So a "one-day" would be a burglar trying to hack a lock one day after Home Depot announced the problem, and a burglar might have a decent chance of breaking in if they picked a lazy or slow home owner's home. A "30-day" would be a lot less likely for the burglar to succeed, since most home owners would have hopefully fixed their lock by then.

A "zero-day" would be if the burglars found out first before Home Depot did. Then any burglar who knows about the hack could break into the home of anyone who owns that lock, since no one would have fixed it.