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The Dates on Your Food Are Mostly Guesses
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When the Sahara Was Green
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Why Old Songs Sound "Warmer"
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Grab your coffee and let’s dive in.

In the United States, almost none of the dates printed on your groceries have anything to do with safety, and apart from infant formula, no federal law requires them at all. The "sell by" date is not even meant for you. It is a note from the manufacturer to the store, telling the shop how long to keep the product on display, which is why milk kept properly in the fridge usually lasts five to seven days past the sell by date before it ever turns sour. The "best by" and "use by" dates are a little more useful, but only just, since they mark the manufacturer's best guess at when the food will taste freshest, not when it becomes dangerous. They are predictions about flavor and texture, made by a company that has every reason to err on the cautious side.

This confusion has a real cost. The US Department of Agriculture estimates that around 30 percent of the food supply is lost or wasted, and a large share of that comes from people tossing perfectly good food because they misread a quality date as a safety deadline. Eggs stay good for three to five weeks past their sell by date if refrigerated, an unopened can with no dents or bulges can be safe for years, and dried goods like pasta, rice, and flour sit happily in the cupboard long after their best by date, growing stale rather than hazardous. The food industry has noticed the problem, and groups representing manufacturers and retailers have been pushing to standardize the chaos down to just two phrases, "best if used by" for quality and "use by" for the genuinely perishable.

So if the printed date is not the real authority, what is? Your own senses are far better spoilage detectors than a number stamped on a package weeks or months earlier. Food safety experts say the most reliable test is simply to look, smell, and taste, because the signs of true spoilage are usually obvious once you know to check for them. White, green, or black mold is a clear stop, especially on soft or porous foods like bread, where the thread-like roots spread invisibly through the whole loaf and mean the entire thing should go. A sour or off smell, a slimy texture on meat, or a color that has turned all point to food that has actually gone bad, regardless of what the label claims. The one place to defer to the date rather than your nose is baby formula, whose use by date really is about safety and really should be obeyed. For nearly everything else, the printed date is a suggestion, and the sharper instrument is the one attached to the front of your face.

The largest hot desert on Earth, a sea of sand and stone where rain almost never falls, was a lush green grassland dotted with lakes just a few thousand years ago. Between roughly 11,000 and 5,000 years ago, in a stretch scientists call the African Humid Period, the Sahara was a savanna much like the plains of modern Kenya, crossed by flowing rivers and holding permanent lakes, some of them enormous. Where there is now nothing but dunes, in what is today Libya, Niger, Chad, and Mali, there were hippos, elephants, giraffes, and crocodiles, along with people who hunted, fished, and herded cattle across the greenery. We know this because they left their record on the rock, painting and carving scenes of swimming figures and grazing herds onto cliffs that now stand in some of the driest, most lifeless places on the planet. The desert is, in a sense, haunted by its own wetter past.

The cause of all this was the tilt and wobble of the Earth itself. Our planet does not spin perfectly steadily, and over tens of thousands of years its orbit and the angle of its axis slowly shift in regular patterns known as Milankovitch cycles. These changes alter how much summer sunlight reaches the Northern Hemisphere, and during the green period that extra solar energy supercharged the West African monsoon, dragging heavy seasonal rains far north into what is now desert. Once the rains arrived, the land helped itself. Grass and trees took root, and vegetation holds moisture and draws in more rain, a feedback loop that kept the Sahara green for thousands of years. The whole region swings between desert and grassland on a cycle of roughly 20,000 years, and this has happened again and again throughout the time of human evolution, so the green Sahara was not a fluke but one turn of a very slow wheel.

The ending, when it came, was startlingly fast. As the orbital cycle moved on and the summer sunlight faded, the monsoon weakened, and the same feedback that had kept the land green went into reverse, with dying vegetation leading to less rain and less rain killing more vegetation. The transformation from savanna back to desert took only a couple of centuries, an abrupt collapse that drove people and animals out and left the Sahara as the barren expanse we know. Scientists project that the orbital conditions needed to green the Sahara will next line up roughly 10,000 years from now, around the year AD 12000, when the rains could return and the desert could bloom once more. The one great uncertainty is us, since no one is yet sure how the carbon we are pumping into the atmosphere will interfere with a cycle that has been turning the Sahara green and brown since long before humans existed.