UA 4258 then AC 8539 at Dulles
Cincinnati Northern Kentucky → Washington → Toronto (YTZ)
Timetable, Washington time
21:22UA 4258 from Cincinnati Northern Kentucky at the gate
17:20AC 8539 to Toronto (YTZ) leaves the gate
19 h 58 apart on the timetable, overnight
AC 8539 leaves 19 h 58 after UA 4258 arrives on the timetable: more than half a day, so not a connection we count.