He also said another vote on Brexit would be unfair, and reiterated that he would like to see a US trade deal with the United Kingdom after it leaves the EU.
Asked whether he thinks the United Kingdom should push back its exit date, Trump said: "I think they're probably going to have to do something, because right now they're in the midst of a very short period of time - the end of the month - and they're not going to be able to do that". "I don't think another vote would be possible because it would be very unfair to the people that won", Trump said.
"We have a different opinion, president", Irish Prime Minister Leo Varadkar said mildly.
Gardiner rejects Irish concerns about the border and said the issue has been used as a "battering ram" by European Union leadership keen to make Britain a cautionary example to other member states flirting with an exit. "I stand here leader of my country, flawed and human, but judged by my political actions and not my sexual orientation, my skin tone, gender, or religious beliefs", he said.
Trump has previously been critical of May's Brexit deal, warning that it could "kill" a bilateral trade agreement between the USA and the UK.
Varadkar and his partner are in Washington as part of an annual trip ahead of St. Patrick's Day. "I regret that Brexit's happening". Mr Trump said the "issue on the Border of Ireland is one of the most complex points".
Ireland is at the center of the practical discussion of how Britain would separate itself from the European Union and its open movement of goods and people, because the only affected land border runs between Ireland and Northern Ireland.
Last night, he presented the US President, Donald Trump, with the traditional bowl of shamrock in a ceremony at the White House. "I predicted it was going to happen".
She told BBC News NI that she would "very much" like to se the U.S. president visit Northern Ireland. "He has consistently emphasized Brexit as a prerequisite for a new U.S. -U.K. free trade agreement, and administration officials and members of Congress have been working diligently to prepare for such an agreement", Gardiner, director of the Margaret Thatcher Center for Freedom at The Heritage Foundation, tells VOA News.
Turning to Mr Varadkar, he said: "Leo, I'm sure you agree on that".
"What I've asked for is an understanding of our situation, particularly when it comes to Northern Ireland and avoiding a hard Border and protecting the peace process".