lisati:remain focused on the problem.
The 'Date' header is a header, meaning it is created by the MUA (software) that created the email in the first place. It can't be trusted as in any way reliable. Most common scenario is that the computer generating the spam has its local clock set wrong. Second most common would be that the emails are created by a program or script that inserts a bollocks date: header and does't create a real one at all. When receivers sort their mail by the date header, sometimes doing odd things actually helps gain viewers (and thus clicks).
Agreed, the Date header can be messed with, as is the case with most of the other information we can see in other headers. I've seen email with apparent time stamps that occur after the time I'm actually reading it.
About the only headers I can trust completely are the ones added by the software I'm using.