Havdalah Times
A few months ago, the OU modified its policy for displaying the time for havdalah on the Candlelighting page. The time displayed is 42 minutes after sunset.
The interval between candlelighting (18 minutes before sunset) and havdalah (42 minutes after sunset) is therefore EXACTLY one hour. The only exception in the list of cities is Jerusalem, for which candlelighting is 40 minutes before sunset, and havdalah is 42 minutes after sunset.
There are many customs regarding the calculation of nightfall to determine when the Sabbath ends and weekday work can be resumed. These calculations of nightfall range from the time of the appearance of three smaller stars, calculated as the time when the sun is 8.5 degrees below the horizon, to the calculation of Rabbeinu Tam that nightfall begins 72 minutes after sunset.