system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can a diary be admissible as evidence? Couldn't it be completely forged the night before?

I see diaries often mentioned as great tools for keeping organizations honest and a few big political cases where diaries or personal journals became major evidence.

Is it just so the journal-keeper can keep better track of his or her own memories better or are they validated in some way?

edit: The ELI5 gist I'm getting is that it's usually considered something along the lines of a verbal testimony, and not usually used by itself to decide a case, but that there are a lot of variables involved such as how important the diary evidence might be, and it has the added weight that it's already written and cannot be changed later. And there are ways to at least validate that it was written when it was claimed to have been written.

Thanks for the answers guys!
gpt
They can be evidence as much as any other written document can be. Yes, a diary could have been forged. But so could any other written document. Just like a witness might be lying, computer evidence falsified or any other kind of evidence corrupted. Evidence has to be critically examined to ensure that, as best as can be determined, it's genuine. 

For example, if the relevant diary entry was in a different hand, different pen and on a page stapled into the rest of the diary, there would be good grounds for assuming it was false.