Asciinema now stores timestamp from the start of the capture. To skip start of the capture, one needs to offset the timestamps from the first kept in the capture.

asciinema_rebase.py

#!/usr/bin/env python

import sys
import json

start_time = None

for line in sys.stdin.readlines():
  o = json.loads(line)
  if type(o) != type([]):
    sys.stdout.write(line)
  else:
    if not start_time:
      start_time = o[0]
    o[0] = o[0]-start_time
    sys.stdout.write(json.dumps(o) + '\n')