Hmm mea culpa for my not reading the small print properly, but there remains a problem, and the slightly counter-intuitive way in which LR works in this regard does not explain it.
For instance, I have a folder containing 48 tiff image files, which all have the same original date (but not time) displayed in the metadata pane: They were all scanned on the same day, thus the original date is (say) 21/01/2014.
If I select all those, and use Edit Capture Time to a specified date and time (say 19/01/2014, so subtracting 48 hours) , they should all change to the same capture date should they not?
But this has not happened - what's happened is that some are set correctly, and some are not, yet they all start out with the same date (but not time) before the adjustment is carried out.
If however I had a folder containing a set of files where there was a mixture of original dates and times, I might expect the sort of result I'm getting ... but I'm not.
Nor does your rationale explain why some dates are pushed forwardand some pushedback.
So I remain unclear on why I am getting the results I am.
It does seem slightly absurd, by the by, that LR does not contain functionality to allow one to change a set of image files to a particular date rather than only by an offset. It's perfectly easily done in Exiftool (either directly or the ExifMeta Exiftool plugin)