Well, the question of how useful {modtime} is if you can't actually use it aside...
I tried your string again this morning on a different batch of videos that I shot on the 22nd and today, the 27th. For all the MXF files from my Canon camcorder and MP4 files from my Hero4 Silver it worked. For the MOV files outta my 1D.4 the date is coming in as today for all of the files. I wouldn't know if I hadn't shot the video on two separate days. I can right click and view the properties and Created is today, the time it was copied to the server, and the Modified time is correct at either the 22nd or 27th. But, PM is filling that field with 27, today, for all of the files.
I switched it up to {moddate} {modtimesort} {filename} and that worked although the date format is 12-27-2017 which makes it exponentially less useful than 2017-12-27.
Strangely, I then for giggles tried {moddate} {modtime} {filename} which also worked. So, it would appear that {modtime} works fine as long as it's not the last variable in the renaming structure.
So I went about trying to find another combination that gave me dashes in the date. Wound back up at {year4}-{month0}-{day0} {modtime} {filename} and the date was now coming in correctly as the 22nd or 27th.
So, that was 45 minutes of my life I won't get back but it appears to be working. For now...