Date and Time Taken (Why is Windows so awful with this?)

[Saltrams]

OK, "Why" isn't really my question. It just IS awful. Question is, how to outwit it? My problem seems simple to me:

I want to change (or add to, where there is no data) the Date Taken field as shown by Win10 explorer (EXIF:DateTimeOriginal, EXIF:CreateDate or XMP:CreateDate?) on an individual file basis (and perhaps later on a whole folder). Background is that these are old images, slide film, scanned prints or similar so there is no truly original date. However, I know what the date should be so I want to write it to the metadata, which ought to be simple for individual files just by using Win Explorer but Win10 is so bleepin' awful it doesn't allow the TIME to be written, only the date. So, when I then try to display files by date taken the files in question are slightly out of order because the time is written as the time that I manually entered the date information using Win Explorer. I could always set myself an alarm so that I can individually write the information at just the right time of day & outwit Win10 I suppose  Duh! Microsoft. So near and yet....)

I can find examples to shift dates and times by certain amounts so I suppose I could do the maths for each time but surely there is an easier command!?

[StarGeek]

I'm not sure what you actually want to do.  You can set the time stamp in the file with this
exiftool -DateTimeOriginal="2020:03:17 08:30:00" <FileOrDir>

As you found, you can shift individual time stamps.

If you want to make the files in an order without worrying about the exact time, take a look at this post.

As for what tags Windows uses for the "Date Taken" property, there are multiple tags, but you don't need to set them all, DateTimeOriginal is usually the best.  You can see what tags Windows reads/writes in this post.

[Saltrams]

Yes! You did understand what I need to do. 
I apologise for the delay in seeing your reply & thanking you for helping; I haven't yet tried the command but I'm sure it's what I need.
Best regards, stay well.