Compute time difference between events

[pranaynanda]

I have events like

Event EndDateTime
Launch 2017-05-16 13:00:00
.
.
.
Open 2017-05-16 13:00:30

I want to subtract time between these two events.

I want to implement something like

index="myindex" sourcetype="mysourcetype"  | transaction host startswith="Launch" endswith="Open"|convert timeformat="%Y-%m-%d %H:%M:%S" mktime(EndDateTime)| eval difference=[subtract EndDateTime where Event=Open - EndDateTime where Event=Launch| chart  avg(difference) 

I just can't understand how can I work with the eval part about calculating difference.