Shift timestamps for captions in VTT files. Usage: python3 captions-shift.py my_captions.vtt my_edited_captions.vtt -2.25

captions-shift.py

#!/usr/bin/env python3

import webvtt
from datetime import datetime, timedelta
import argparse

parser = argparse.ArgumentParser(description="Shift caption start \
				     and end times in a .vtt file")
parser.add_argument("inputfile", help="input filename, must be VTT format")
parser.add_argument("outputfile", help="output filename")
parser.add_argument("seconds", type=float, help="number of seconds to  \
                                                 shift caption times. Can be \
                                                 negative.")

args = parser.parse_args()

def adjust_time(timestamp, seconds):
    t = datetime.strptime(timestamp, "%H:%M:%S.%f")
    d = timedelta(seconds=seconds)
    new_timestamp = t + d
    
    return new_timestamp.strftime("%H:%M:%S.%f")

vtt = webvtt.read(args.inputfile)

for caption in vtt:
    caption.start = adjust_time(caption.start, args.seconds)
    caption.end = adjust_time(caption.end, args.seconds)

vtt.save(args.outputfile)

Glad it was useful to someone else!