You should be able to rely on @timestamp being a Time.

Your options for getting a unix epoch are:

1. `event.sprintf("%{+%s}")` - gets you unix epoch in seconds as a string
2. `event["@timestamp"].to_i` - gets you unix epoch in seconds as an integer
3. `event["@timestamp"].to_f` - gets you unix epoch in seconds as a float (with subsecond precision)