Early Departures are my forte  :roll:  due to the difference in time zones, and my early morning work schedule (4:00 am server time).  I am subject to departure between 8:00pm (earliest) and 9:30 pm (latest) server time on nights that I work the following day.  

I usually announce that I have to leave, and stay to complete the boss or encounter that we are on is completed.

Additionally, If I'm not on for the start of a raid, it usually means that I have RL aggro  :) , and will be unable to attend.

The following has worked well for me: 
Ask about invites about 30 minutes before the start time of the raid, obtain any gear / consumables that I need, and when invited, travel to the raid site at 15 min to start time.