字幕文件时间偏移

有一些视频资料的字幕不同步，需要批量修改时间，写了如下脚本：
内容如下
00:00:06.400 --> 00:00:08.700
Bypassing Network Filters
比如延迟一秒：
00:00:07.400 --> 00:00:09.700
Bypassing Network Filters

# coding:utf-8
import os
import re
import datetime


def listFiles(dirPath):
    fileList = []
    for root, dirs, files in os.walk(dirPath):
        for fileObj in files:
            fileList.append(os.path.join(root, fileObj))
    return fileList


def main():
    # fileDir = "/home/huanmsf/Documents/test/"
    fileDir = "/home/huanmsf/Documents/PEN300/media/captions/"
    fileList = listFiles(fileDir)

    for fileObj in fileList:
        f = open(fileObj, 'r+')
        all_the_lines = f.readlines()
        f.seek(0)
        f.truncate()
        for line in all_the_lines:
            # str1 = 'original string'
            # str2 = 'replaced string'
            # f.write(line.replace(str1, str2))
            print(line)
            date_all = re.findall(r"(\d{2}:\d{2}:\d{2}\.\d{3})", line)
            if len(date_all) > 0:
                a1 = datetime.datetime.strptime(date_all[0], "%H:%M:%S.%f")
                astr1 = (a1 + datetime.timedelta(seconds=5)).time().__str__()
                if len(astr1) > 12:
                    astr1 = astr1[:-3]
                if len(astr1) == 8:
                    astr1 = astr1 + ".000"
                a2 = datetime.datetime.strptime(date_all[1], "%H:%M:%S.%f")
                astr2 = (a2 + datetime.timedelta(seconds=5)).time().__str__()
                if len(astr2) > 12:
                    astr2 = astr2[:-3]
                if len(astr2) == 8:
                    astr2 = astr2 + ".000"
                timePart = astr1 + " --> " + astr2 + "\n"
                print("=======================>")
                print(timePart)
                f.write(timePart)
            else:
                f.write(line)
        f.close()


if __name__ == '__main__':
    main()