Amazon's promises may not be exactly what customers expect, if history is anything to go by.

The company announced during an earnings call last Thursday that it will be transitioning its Prime two-day shipping guarantee into a one-day shipping guarantee.

The announcement sent most of the affected parties into a whirlwind of speculation. Walmart even responded with a vague promise to match Amazon's new speed.

Some customers responded to Amazon's news with a request: perfect two-day shipping first, before worrying about expanding one-day.

"Seriously?" one commenter wrote on Facebook. "A third of the time they can't even get my packages to me in two days. Sometimes my Prime packages take five days or they get lost and don't show up at all."

Read more: Amazon is promising one-day Prime shipping — but skeptical customers are sounding the alarm

These customers were skeptical of Amazon's ability to deliver on a new promise, noting that their packages with two-day shipping often don't arrive within two days of order.

In fact, this is a complaint that Amazon customers have been levying against the company for years now.

The complaints reveal Prime's limitations and how different the two-day-shipping guarantee is from what some customers perceive it to be.

Two-day shipping ensures only that customers will get it within two days from the time it's handed over to the carrier, not within two days from the time of ordering, which is often incorrectly assumed.

"If the item you're ordering is out of stock or unavailable to ship immediately, you may not receive the item in two business days. Selecting One-Day or Two-Day shipping will reduce the transit time to one or two business days after we've shipped your order, but it won't impact how long it takes us to obtain the item or prepare it for shipment," the policy reads on Amazon's website. "The shipping method time starts when the item ships."

Many items do, however, ship immediately and are at a customer's doorstep in two days, which may have created this customer expectation.

"What they've done is set an expectation for Prime ... that people would get the items they ordered very quickly, and certainly within two days typically of order," Brandon Muramatsu, a frequent Amazon customer, told Business Insider in 2018.

For customers confused by all of this, there's an easy way to tell when a package will arrive. Look for the "guaranteed delivery" date on the last page before finalizing the order. That will eliminate any surprises.