When does snail mail get post-dated?

Question

In other words, if my postal carrier picks up my out-going mail from my place of residence, does that mail get post-dated that same day? Or does it get post-dated when the post office gets around to it, such as, by the next day?

Correction: I meant "postmarked." :-P

Answer 1

I believe you mean postmarked.
Your mail will be postmarked the same day that it is picked up at your residence by your carrier.
If you were to mail it at a blue collection box before the last pick-up time listed on the box, it would also be postmarked that day. If you were to drop it in that box after the latest scheduled time it would be postmarked on the next day that mail is scheduled to be picked up from that location.

Answer 2

Snail mail as you say, gets a post mark for two reasons, one is that it cancels the use of the stamp for future use, and that it has been handle by which post office on what date.
The post mark is put on the letter as soon as it hit the post office, because now the post office has machines that are automated to speed up the process. It is put in to these sorters to allow it to be put in a bag for handling to be sent to it place of destination. So even if your mail gets picked up after 6pm it still get the post mark of that day.