Morning Extra magic hours are always one hour prior to official park opening time... unless you are coming during a holiday where they have changed the opening time, all parks open at 9am everyday so the EMH would start at 8am.
For evening extra magic hours the park is open for 3 additional hours past the official park closing time so that could vary based on time of year and even day to day, depending on what time the park closes... In general AK usually closes around 5 or 6 pm so the EMH would be from 5-8pm (6-9pm) just depends on the closing time when you are visiting.