Before opening this thread i used the searchbar and it led me to a very similar problem
In the case above the user wanted to fill datetaken with datecreated, instead i mean to use lastmodified as my source.
Unlike the user in that thread the change timestamp tool does not help me complitley since it doesn't work when the jpg file i'm working with does not have a date taken value
The checkbox "EXIF: Date Taken" inside the "Change Timestamp" groupbox is blurred, but it shows that it would edit to the correct value (it is selected because of a previous operation on a jpg that had a datetaken value)
I considered using jhead but it doesn't seem to have a suitable command for it since i need it batch processed for 5k+ jpg files.
I Assume the problem is with the exif data attached to these files but i don't know enough about the matter.