Awk program to correct the timing of a srt subtitle file by applying an offset.

offsetsrt.awk

function string_to_time(time_string) {
  split(time_string, s, ",")
  split(s[1], t, ":")
  return 3600*t[1] + 60*t[2] + t[3] + s[2]/1000
}

function correct_time(time_string) {

  offset = -16.0
  start_time = string_to_time("00:00:00,000")
  end_time = string_to_time("23:59:59,999")

  time = string_to_time(time_string)
  if(time < start_time || time > end_time)
    return time_string
  time += offset 

  hour = int(time/3600)
  min = int(time/60) % 60
  sec = time % 60
  ms = 1000 * (time % 1)

  return sprintf("%02i:%02i:%02i,%03i", hour, min, sec, ms)
}

m = /^[0-9]{2}:[0-9]{2}:[0-9]{2},[0-9]{3} -->/ {
  printf("%s --> %s", correct_time($1), correct_time($3))
  for (i=4; i<NF; i++) { printf(" %s", $i) }
  printf("\n")
}

!m {
  print($0)
}