David,
If you convert to TIF or JPEG, and then use Opus to add or change the Windows property 'Date Taken', that date will be written to enough metadata fields that the date you enter will be recognized by applications as the 'shutter clicked date' - and i think that's your goal.
I like that you wrote 'shutter clicked' date'. I use the term 'Snap Date' when referring to when the shutter was clicked whether that date and time is know or unknown.
If you really want to see EXIF-ModifyDate, install the ExifTool Custom Columns plugin:
Leo mentioned a 'new field'. I think he is referring to the recently-added 'Date/Time Created' field. The release notes did not specify if it's an EXIF field, an XMP field, or an IPTC field. I think it's IPTC-DateCreated, but I'm not sure.
If you have other questions about photo metadata, feel free to message me.