On September 11, 2001, the world came to a standstill as America suffered its worst ever terror attack. Two planes ploughed into the World Trade Center in New York City, one hit the Pentagon, and a fourth was heroically steered away from its target by passengers on board the flight.
2,977 innocent victims died as a direct result of the attacks, and countless thousands of families were torn apart by the aftermath.
As a mark of respect for those who lost their lives that day, 9/11 has become a day for mourning and remembrance. Across the globe, people observe a minute's silence for the dead, and - in more recent years - many folks have taken to social media in order to send out messages of commemoration for those who are no longer with us because of that fateful day.
Of course, everyone will have their own way of paying tribute to the deceased, but there are certain unwritten rules about what is and what is not respectful when speaking about 9/11.
Barack Obama's tweet today, for example, appears to show that he understands the true gravity of the loss that occurred 17 years ago, and that he is truly mournful for those who were affected by the attack.
On the contrary, Trump's response appears a little lacklustre, to say the least.
While the former president posted a detailed, nuanced message thanking those who helped and honouring those who died, the current president sent out a five-word tweet which basically just informed people what date it was.
Of course, Trump has made numerous Twitter faux pas in the past, but perhaps none have incited as much outrage in such a short space of time as this one.
Other people have also taken the opportunity to remind Trump of all the terrible lies he has shared about 9/11 over the years.
At one time, he claimed to have been present at ground zero on the day of the attack - and even that he helped the rescue efforts. This was proven to be false.
He also claimed that he had lost "hundreds" of friends when the towers fell. He has never been able to name a single one.
Perhaps worst of all, the former businessman falsely stated that the Arabs in New Jersey "cheered" when the World Trade Centre was hit, despite there being no proof whatsoever that any such thing ever happened.
Over the past 17 years, President Trump has consistently shown disrespect and disregard for the almost 3,000 people who died as a result of an attack on the country that he is now supposed to represent. Meanwhile, Barack Obama has always shared thoughtful, meaningful tributes.
Once again, then, Donald Trump has proven that he is aloof to the concerns and values of not just the average American, but the average person in general. He appears to have no sympathy, no consideration, and no basic recognition for the loss that occurred well within his living memory.
Is his pattern of heartless tweets and reactions to 9/11 a good measurement of who he is as a president? Perhaps. Is it a good indication of who he is as a human being? Almost certainly.
And if Donald Trump does not consider 2,977 victims of terrorism to be worth more than a five-word tweet, how do you think he feels about the rest of us?
