Pompeii, the Roman city preserved for two millennia thanks to a volcanic eruption that engulfed the area in ash in 79 AD, has long been studied as a window to the past. Newly discovered scribbles of graffiti, unearthed by archeologists excavating the site, just changed the historical narrative on precisely when the natural disaster occurred.

The message, written in charcoal on the wall of the house, reads, “XVI K Nov in [d]ulsit pro masumis esurit [ions],” which roughly translates as “he over-indulged in food.” It was written on the wall of a house being renovated, suggesting it could have been scrawled by a builder midway through work. And, crucially, it adds a date: “XVI K Nov,” which refers to the 16th day before the first of November, or October 17, according to a statement from the archeological team behind the discovery, the Parco Archeologico di Pompei.

For centuries, historians believed that the eruption of Mount Vesuvius happened earlier in the year, in August. This was based on a letter from Roman writer Pliny the Younger to the historian Tacitus, written 25 years after the disaster. According to monks’ transcriptions of the original letter, Pliny stated the date was “nonum kal Septembres,” which is nine days before the start of September, or August 24.

A few details about Pompeii made historians suspect that Pliny (or perhaps the monks transcribing copies of his letter) got the date right, but the month wrong. As bioarcheologist Kristina Killgrove wrote in Forbes, archeologists found pomegranates, walnuts, and newly-harvested wine in Pompeii, which suggests it wasn’t destroyed until the autumn. And the warm clothing of those who died at Pompeii would’ve been more appropriate for October than August.

The new discovery adds further evidence that it was autumn when the volcano erupted. As charcoal writing is quickly worn away, archeologists believe the graffiti must have been written within a week of the eruption. “Being charcoal, fragile and evanescent, which could not last long over time, it is more than likely that it is October 79 AD, a week before the great catastrophe that, according to this hypothesis, occurred on 24 October,” they wrote in the statement. This means that on this day, 1,939 years ago, someone idly dawdled a note about their meal on the wall. And, a week later, the entire city was destroyed.