maxhi,
If you have any consistent information to can use this to match data. an example may be to import all the files such as there filename is the tiddler name, then separately use the operating system eg windows Dir command to list the files with created and modified dates along with the filename into a text file. Now import that perhaps as a csv file and use JSON mangler to generate a plugin containing a tiddler (in a plugin) for each file name containing your "original-created and original-modified dates" you can then write a list to look up the dates for each filename and apply it to the tiddler for each text file. We can parse text files as well if necessary.
I strongly urge to to use new fields for dates and retain the integrity of the tiddlywikis created and modified dates. I expect the dates will be in a different format and need conversion anyway, ideally convert them to tiddlywikis date serial number format, even if you have to zero fill the time or milliseconds.
later you can build a method to spilt existing files into each day eg excise
At every step try to retain and import any information you have, it may easily prove useful later, for example if the files were already in folders try and capture the folder name as well. if you need to reformat retain the old and the new values. If some filenames are duplicates (but not their content) there are tricks you can employ.
Of course you will still have some way to go but do lean on us as much as you need, if you can generate some sample or dummy content to share, we can help you further.
Regards
Tones