Always three hours
Pacific Time is 3 hours behind Eastern Time all year, because both regions switch to daylight saving time on the same dates. 9:00 in Los Angeles is 12:00 in New York; a 5 pm deadline in New York is 2 pm in San Francisco.
That makes the difference easy, but the date can still change late in the evening: 10 pm Pacific is 1 am Eastern the next day. The converter shows the date for each city, so schedules across the US stay clear.
Other US zones
Add Chicago for Central Time and Denver for Mountain Time. Phoenix in Arizona stays on standard time all year, so its difference to the other zones changes in summer.