How can I batch-modify timestamp metadata on MP4 video files [closed]

Question 1

Based on the QuickTime File Format Specification I put together a proof-of-concept Python script for crude shifting of the timestamps to get me by for now:

#!/usr/bin/env python
import datetime
import sys

def int_bytes(raw):
    value = 0
    for byte in raw:
        value <<= 8
        value += ord(byte)
    return value

def bytes_int(value, size=4):
    raw = []
    for byte in range(size):
        raw.append(chr((value >> 8*byte) % 256))
    return ''.join(reversed(raw))


ATOM_FORMAT = (
    ('atom_size', 4, int_bytes),
    ('type', 4, str),
    ('version', 1, int_bytes),
    ('flags', 3, int_bytes),
    ('creation_time', 4, int_bytes),
    ('modification_time', 4, int_bytes),
    # that's all I need for now, and is common
    # between tkhd, mvhd and mdhd
    # ...
)

ATOM_TYPES = ('tkhd', 'mvhd', 'mdhd')

TIMESTAMP_EPOCH = datetime.datetime(1904, 1, 1, 0, 0)

def from_timestamp(timestamp):
    return TIMESTAMP_EPOCH + datetime.timedelta(0, timestamp)

def to_timestamp(datetime_obj):
    return int((datetime_obj - TIMESTAMP_EPOCH).total_seconds())

def shift_dates(mp4, atom_type, delta):
    # TODO: refactor
    mp4.seek(0)
    data = mp4.read() # TODO: don't load whole file
    type_index = -1
    while True:
        try:
            type_index = data.index(atom_type, type_index+1)
        except ValueError:
            if type_index < 0:
                raise RuntimeError('Cannot find atom: {}'.format(atom_type))
            else:
                break
        else:
            sys.stdout.write(
                '  Found {} at {}\n'.format(atom_type, type_index))
        offset = type_index - ATOM_FORMAT[0][1]

        header_data = {}
        offsets = {}
        for field, size, convert in ATOM_FORMAT:
            offsets[field] = offset
            offset += size
            header_data[field] = convert(data[offsets[field]:][:size])

        for field in ('creation_time', 'modification_time'):
            original = from_timestamp(header_data[field])
            shifted = original + delta
            mp4.seek(offsets[field])
            mp4.write(bytes_int(to_timestamp(shifted)))
            sys.stdout.write(
                '    {}: {} -> {}\n'.format(field, original, shifted))


if __name__ == '__main__':
    try:
        filename = sys.argv[1]
        days, seconds = map(int, sys.argv[2:])
    except (IndexError, TypeError, ValueError):
        sys.stderr.write(
            "USAGE: {} mp4_file days seconds\n".format(
                sys.argv[0]
            )
        )
        sys.exit(1)

    try:
        f = open(filename, 'rwb+')
    except IOError:
        sys.stderr.write("ERROR: cannot open {}\n".format(filename))
        sys.exit(1)
    else:
        delta = datetime.timedelta(days, seconds)
        sys.stdout.write(
            'Shifting timestamps of {} by {!r}:\n'.format(filename, delta))
        for atom_type in ATOM_TYPES:
            shift_dates(f, atom_type, delta)
        f.close()
        sys.stdout.write('Done.\n')

Question 2

ExifTool now has the ability to edit timestamps and many other tags in MP4/MOV video files. But as the AllDates tag only edits three specific tags, two of which don't appear in video files very often, only one tag in a video file is likely to be edited by the above command (see note about AllDates on the Shortcut Tag Names page).

To edit all time related tags the following command could be used:
exiftool -Time:All+="0:0:729 3:17:0" test.mp4