What the numbers mean
Frank gives Donnie the figure the first time they meet, and he can give it to the second because he is speaking from the other end of the Tangent Universe: Frank is one of the Manipulated Dead, killed on October 30 and reaching back through the whole 28 days. The countdown is not a threat. It is a measurement.
Counted from around midnight on October 2, 28 days, 6 hours, 42 minutes and 12 seconds lands at 06:42:12 on the morning of October 30 - the morning after the Halloween party, as the plane carrying Rose and Samantha Darko flies into the storm over Middlesex and Donnie sends the jet engine back through the Vortex.
The arithmetic
28 days, 6 hours, 42 minutes and 12 seconds is 2,443,332 seconds, or 40,722.2 minutes, or 678.7 hours. The four numbers also add up: 28 + 6 + 42 + 12 = 88 - the year the film is set, and, for anyone counting, the speed a certain DeLorean has to reach. Richard Kelly has never confirmed the second reading, which has not stopped anyone.
The film does not show a clock at the moment of the rupture, so the start time is an inference: Frank wakes Donnie in the small hours, after the family has gone to bed, and the engine falls while Donnie sleeps on the golf course. This site counts from midnight for a clean calendar; the true start could be an hour or two later, which moves the end by the same amount.
How this clock works
The clock above follows the film’s calendar in your local time. From midnight on October 2 until 06:42:12 on October 30 the Tangent Universe is open and the clock counts down to the collapse. Outside that window it counts down to the next October 2 - the next rupture. On October 30 itself, if you are up at 06:42 in the morning, you will see it reach zero.
Below it you can start a countdown of your own. It runs for exactly 28 days, 6 hours, 42 minutes and 12 seconds from the moment you press the button, is stored only in your browser, and can be reset at any time. What you do with your 28 days is, as ever, up to you.