How can people send me spam emails from the future?

That's not an "exploit", rather the way e-mail works.

Datetime, sender, receiver, and all other headers of an e-mail message can be set by the sender to whatever value he wishes; mail protocols make no security check on them. Hence, spoofing the sender of an e-mail (as spammers, scammers, and phishers often do) it's a child's play.

As Priyank correctly said, if you look at the full headers of the message you received you'll see that only the first hop (the sender) bears a date in the future; all the other hops (the MTAs between the sender and you) are correctly timestamped with the actual date.

It is kind of possible.
Suppose you change your system date/time and use a local client like Outlook to send an email, then the receiver will see the changed time. But in the email headers, the actual time of the email when it was received by the server, will be mentioned.
But if you are using some email service on your browser, then the actual time will be seen by the receiver.

Another possibility, which would be EXTREMELY rare, is that the time on the mail server is wrong.

In your case, try checking the headers of the spam emails.

Most messaging systems do allow drag-in/out copying of emails from the client to the desktop. So if you want to send spams with any dates you want, there are various options.

Actually you can quickly code a mass mailer in Python and change automatically whatever parameter you want in the headers, including time (how to change email's header programmatically)

An other option is that you keep your previous mass mailer in Python (I mention Python because I experienced it a lot and it fits very well for such situations) and execute it in Windows OS virtual machines and change the system date automatically to whatever you want so that it will appear in the header of your spams (how to change system date and time programmatically)