Once upon a time when I studied law, the post stamped date determines the date of acceptance, not the date of receipt. That's if one actually can read the stamped date on the envelop and the mail is delivered promptly without undue delay like a year later or never.
I don't know what is the rule for the US Election, it seems like people make up the rule as they go. My initial impression was the ballot mailed on Election date should be counted even it came after the Election date. Mail in ballots are a little mess given that plenty of people can't follow instructions, mostly people do not sign the envelop or cross the ovals instead of filling the ovals . . . . When one casts the ballot on site, those errors can be avoided or corrected right away. If you take any public exam in multiple choice format, those answers would be discarded as you can't follow the instructions. I don't know how those ballots would be treated like without signature or marking it kind of wrong. Or "lost in mail" or "stuck in the post office" or simply delivered days late or no clear readable postal stamped date on it.
Perhaps these errors, if any, may not be statistically significant to be important to the outcome of the race.
