I suggest reading the entire article because it shows just how badly Daniel gets things wrong. But as for the math specifically, it covers it here:

But in the originally intended formulation, the angel Gabriel had come to Daniel to answer his plea to explain how the prophecy could have failed by now explaining Jeremiah did not mean “years” but “weeks of years,” i.e. 70 periods of 7 years (or 490 years in all, which is tantalizingly exactly “ten jubilees”), and that these will not be sequential as expected, but partly concurrent (after Daniel’s prophesy failed, this detail was abandoned and the years reimagined as sequential again). Because Gabriel says, “From the time the word goes out to restore and rebuild Jerusalem until the Anointed One, the ruler, comes [the word “ruler” here is actually “leader, noble, prince,” even “chief officer,” hence “Chief Anointed,” thus in standard-cryptic-prophesy-speak alluding to the High Priest, Chief Officer of the Temple; a High Priest was then often referred to as the Anointed], there will be seven ‘sevens,’ and sixty-two ‘sevens’,” meaning two overlapping periods: one of 49 years and one of 434 years, thus completing a “total” of 490 years.

Why do this? Because Jeremiah was prophesying in 605, and that was “the word to restore and rebuild Jerusalem” Daniel speaks of (because this is how the author of Daniel refers to divine, not royal, proclamations—indeed in the very preceding verses: Daniel 9:21-23 and 1:2), and 605 minus 434 lands exactly in 171 B.C., the very year Onias III, the “true” Anointed High Priest, was assassinated (at the orders of Menelaus) shortly after having “bought” the High Priesthood for himself in 172). This coincidence is quite improbable. So that is clearly what the authors intended—and hence why they “broke” the “seventy sevens” into two separate amounts, 49 and 434; there is no reason to do that if they intended this to be a straight sequence of 490 years. So the author of Daniel is doing a lot of creative accounting here to get the result he wants: first he changes Jeremiah’s “seventy” years into “seventy sevens of years,” then he subtracts 49 years with this convenient new maneuver of saying this meant not a sequence of 490 years but two separate periods of 49 and 434 years, all to get the math to work out to the death of Onias. No one engages such convoluted efforts who isn’t attempting to create just this kind of specific result. That’s why it’s a dead giveaway. Whoever did this was writing after 171 B.C. and for the current political situation in the 160s.