Mailed stamp with date

You're referring to a postmark!

A postmark is a mark or stamp applied to an envelope or package by a postal service to indicate the date and location of mailing. It's a way for the postal service to record the date and time when the item was sent.

Here's an example of what a postmark might look like:

[City, State, ZIP] [Date]

For example:

NEW YORK, NY 10001 FEB 22 2023

In this example, the postmark indicates that the envelope was mailed from New York, New York on February 22, 2023.

Postmarks can be found on the back of an envelope, usually in the top right corner. They're an important part of the postal process, as they help track the movement of mail and provide a record of when it was sent.