I might be late to this party, but i wrote a python script for bulk changing taken time field for whatsapp photos based on filename format Eg: IMG-20160117-WA0001.jpg.
Also this does not overwrite the existing properties. https://github.com/dsouzawilbur/Scripts/blob/master/Change_Photo_Taken_Time.py
from datetime import datetime
import os
import re
import piexif
def absoluteFilePaths(directory):
    for dirpath,_,filenames in os.walk(directory):
        for f in filenames:
            fullPath = os.path.abspath(os.path.join(dirpath, f))
            if re.match(r"^IMG-\d\d\d\d\d\d\d\d-WA\d\d\d\d.*", f) and not re.match(r"^IMG-\d\d\d\d\d\d\d\d-WA\d\d\d\d-ANIMATION.gif", f):
                print(f+" Matched")
                match = re.search("^IMG-(\d\d\d\d)(\d\d)(\d\d)-WA\d\d\d\d.*", f)
                year = match.group(1)
                month= match.group(2)
                day = match.group(3)
                exif_dict = piexif.load(fullPath)
                #Update DateTimeOriginal
                exif_dict['Exif'][piexif.ExifIFD.DateTimeOriginal] = datetime(int(year), int(month), int(day), 4, 0, 0).strftime("%Y:%m:%d %H:%M:%S")
                #Update DateTimeDigitized               
                exif_dict['Exif'][piexif.ExifIFD.DateTimeDigitized] = datetime(int(year), int(month), int(day), 4, 0, 0).strftime("%Y:%m:%d %H:%M:%S")
                #Update DateTime
                exif_dict['0th'][piexif.ImageIFD.DateTime] = datetime(int(year), int(month), int(day), 4, 0, 0).strftime("%Y:%m:%d %H:%M:%S")
                exif_bytes = piexif.dump(exif_dict)
                piexif.insert(exif_bytes, fullPath)
                print("############################")
absoluteFilePaths("__DIRECTORY_WITH_PHOTOS__")