The IRS "post-dates" many notices to account for delays in processing and delivering them to taxpayers.
IRS notices often have a deadline (e.g., 15 days, 30 days) for taxpayers to respond to the notice. The IRS doesn't want you to lose any of those days because your mail is delayed or you aren't home when the notice come, so they use a future date on the notice to delay the start of the deadline.